If AACRH32_SP is not specified, it causes the DEFAULT_LINKER_SCRIPT
variable to be empty, and then the linker takes the variable following
it as if it was the linker script, which is not one. This patch
addresses that issue by requiring the AARCH32_SP variable to be set
before continuing.
Change-Id: I21db7d5bd86b98faaa1a1cd3f985daa592556a2d
Signed-off-by: Juan Pablo Conde <juanpablo.conde@arm.com>