buildsystem: add BUILD_FLAG support

- replace strip_lto/strip_gold (only allowed to disable)
- add flag for PIC feature
- add flag to stop build parallel
- add support for hardening option (initial copy from debian 9)

All build parameters, are added in setup_toolchain.
`PKG_[FLAG]_[HOST/TARGET]_ENABLED` variable is introduced for checking the flag (yes/no) in the package.mk

Thanks to @MilhouseVH, for support and fixing

config/functions

@@ -1,4 +1,44 @@
 setup_toolchain() {
+  # lto flag
+  if flag_enabled "lto" "$LTO_SUPPORT" "only-disable"; then
+    export TARGET_CFLAGS+=" $CFLAGS_OPTIM_LTO"
+    export TARGET_CXXFLAGS+=" $CXXFLAGS_OPTIM_LTO"
+    export TARGET_LDFLAGS+=" $LDFLAGS_OPTIM_LTO"
+  fi
+
+  # gold flag
+  if flag_enabled "gold" "$GOLD_SUPPORT" "only-disable"; then
+    export TARGET_LDFLAGS+=" $LDFLAGS_OPTIM_GOLD"
+  fi
+
+  # position-independent code
+  if flag_enabled "pic" "no"; then
+    export TARGET_CFLAGS+=" $CFLAGS_OPTIM_PIC"
+    export TARGET_CXXFLAGS+=" $CXXFLAGS_OPTIM_PIC"
+    export TARGET_LDFLAGS+=" $LDFLAGS_OPTIM_PIC"
+  fi
+  if flag_enabled "pic:host" "no"; then
+    export HOST_CFLAGS+=" $CFLAGS_OPTIM_PIC"
+    export HOST_CXXFLAGS+=" $CXXFLAGS_OPTIM_PIC"
+    export HOST_LDFLAGS+=" $LDFLAGS_OPTIM_PIC"
+  fi
+
+  # hardening support
+  if flag_enabled "hardening" "$HARDENING_SUPPORT"; then
+    export TARGET_CFLAGS+=" $CFLAGS_OPTIM_HARDENING"
+    export TARGET_CXXFLAGS+=" $CXXFLAGS_OPTIM_HARDENING"
+    export TARGET_CFLAGS+=" $CPPFLAGS_OPTIM_HARDENING"
+    export TARGET_LDFLAGS+=" $LDFLAGS_OPTIM_HARDENING"
+  fi
+
+  # parallel
+  if ! flag_enabled "parallel" "yes"; then
+    NINJA_OPTS="$NINJA_OPTS -j1"
+    export MAKEFLAGS="-j1"
+  else
+    export MAKEFLAGS="-j$CONCURRENCY_MAKE_LEVEL"
+  fi
+
   case "$1" in
     target|init)
       export DESTIMAGE="target"
@@ -318,6 +358,37 @@ listremoveitem() {
   echo "${tmp_array[@]}"
 }
 
+# check if a flag is enabled
+# $1: flag-name, $2: default (yes/no), $3: ingenious check (none,only-disable,only-enable)
+# set variable PKG_[FLAG]_[HOST/TARGET]_ENABLED=(yes/no)
+# return 0 if flag is enabled, otherwise 1
+flag_enabled() {
+  # make flag name upper case and replace hyphen with underscore, to use as variable name
+  local flag_name=${1^^}
+  [[ $flag_name =~ : ]] || flag_name+="_TARGET"
+  flag_name="PKG_${flag_name//[:-]/_}_ENABLED"
+
+  # check flag
+  if [ -n "${PKG_BUILD_FLAGS}" ] && listcontains "${PKG_BUILD_FLAGS}" "[+]?$1"; then
+    if [ "${3:none}" = "only-disable" ]; then
+      echo "ERROR: $1 can not enable via PKG_BUILD_FLAGS (found in $PKG_NAME)"
+      exit 1
+    fi
+    declare ${flag_name}="yes"
+    return 0
+  elif [ "$2" = "yes" ] && ! listcontains "${PKG_BUILD_FLAGS}" "-$1"; then
+    declare ${flag_name}="yes"
+    return 0
+  else
+    if [ "${3:none}" = "only-enable" ]; then
+      echo "ERROR: $1 can not disable via PKG_BUILD_FLAGS (found in $PKG_NAME)"
+      exit 1
+    fi
+    declare ${flag_name}="no"
+    return 1
+  fi
+}
+
 target_has_feature() {
   listcontains "$TARGET_FEATURES" "$1"
 }
@@ -662,26 +733,6 @@ do_autoreconf() {
   fi
 }
 
-strip_lto() {
-  # strip out LTO optimization from *FLAGS
-  if [ -n "$GCC_OPTIM_LTO" ] ; then
-    CFLAGS=`echo $CFLAGS | sed -e "s|$GCC_OPTIM_LTO||g"`
-    CXXFLAGS=`echo $CXXFLAGS | sed -e "s|$GCC_OPTIM_LTO||g"`
-    TARGET_CFLAGS=`echo $TARGET_CFLAGS | sed -e "s|$GCC_OPTIM_LTO||g"`
-    TARGET_CXXFLAGS=`echo $TARGET_CXXFLAGS | sed -e "s|$GCC_OPTIM_LTO||g"`
-  fi
-  if [ -n "$LD_OPTIM_LTO" ] ; then
-    LDFLAGS=`echo $LDFLAGS | sed -e "s|$LD_OPTIM_LTO||g"`
-    TARGET_LDFLAGS=`echo $TARGET_LDFLAGS | sed -e "s|$LD_OPTIM_LTO||g"`
-  fi
-}
-
-strip_gold() {
-  # strip out usage from GOLD linker
-  LDFLAGS=`echo $LDFLAGS | sed -e "s|-fuse-ld=gold||g"`
-  TARGET_LDFLAGS=`echo $TARGET_LDFLAGS | sed -e "s|-fuse-ld=gold||g"`
-}
-
 fix_module_depends() {
   # modify .modinfo section in kernel module to depends on other required modules
   local MODULE="$1"