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

packages/x11/lib/libXau/package.mk

@@ -27,9 +27,6 @@ PKG_DEPENDS_TARGET="toolchain util-macros xproto"
 PKG_SECTION="x11/lib"
 PKG_SHORTDESC="libXau: X authorization file management libary"
 PKG_LONGDESC="X authorization file management libary"
+PKG_BUILD_FLAGS="+pic"
 
 PKG_CONFIGURE_OPTS_TARGET="--enable-static --disable-shared --enable-xthreads"
-
-pre_configure_target() {
-  CFLAGS="$CFLAGS -fPIC -DPIC"
-}