Similar to bl31 allow sp_min to also include a platform-specific
linker script. This allows for example to place specific code in
other memories of the system, like resume code in sram, while the
main tf-a lives in ddr.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Change-Id: I67642f7bfca036b5d51eb0fa092b479a647a9cc1