build(makefile): add extra makefile variable for extension

Introduce EXTRA_LINKERFILE for GCC linker options. GCC linker
can realize multiple linker scripts, and vendors can extend ro or
text sections by inserting sections among the original sections
specified by blx.ld.S.

Vendors can assign compiled object files by assigning MODULE_OBJS
with their own built path.

Change-Id: I1bd2e0383a52204723816131da4b7948def4c4e9
1 file changed