Introduce a new linker flag LDFLAGS_FINAL

Introduce a new linker flag LDFLAGS_FINAL

commit 8aba9dceebb14144e07d19593111ee3a999c37fc
Divides variable of linker flags to LDFLAGS-u-boot and LDFLAGS

breaks the usage of --gc-section to build nand_spl. We still need linker option
--gc-section for every uboot image, not only the main one. LDFLAGS_FINAL passes
the --gc-sections to each uboot image.

To get the proper linker flags, we use LDFLAGS and LDFLAGS_FINAL to replace
PLATFORM_LDFLAGS in the Makefile of each nand_spl board.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Haiying Wang <Haiying.Wang@freescale.com>