treewide: refactor to use PKG_BUILD_FLAGS:=gc-sections

treewide: refactor to use PKG_BUILD_FLAGS:=gc-sections

See commit da370098 "treewide: add support for "gc-sections" in
PKG_BUILD_FLAGS" on the main repository.

Note: This only touches packages which use all three parts
(-ffunction-sections, -fdata-sections and -Wl,--gc-sections) enabled by
this build flag. Some packages only use a subset, and these are left
unchanged for now.

Signed-off-by: Andre Heider <a.heider@gmail.com>