Use variable PLATFORM_ADDITIONAL_TARGET to allow define machine
dependent atf targets. This avoids checking ${MACHINE} in shell
script, and allows define additional build target using overrides.
For exmaple:
PLATFORM_ADDITIONAL_TARGET ??= ""
PLATFORM_ADDITIONAL_TARGET_ls1012afrwy = "ls1012afrwy_512mb"
PLATFORM_ADDITIONAL_TARGET_xxx = "yyy" # for other board
