[TESTS] Fix basic firmware Makefile.inc for native compilation

For native compiling basic firmware, the CROSS_COMPILE environment
variable will not be set hence we should not assume any cross-compile
prefix in basic firmware Makefile.inc when CROSS_COMPILE is not defined.

This patch patch fixes basic firmware Makefile.inc as-per above.

Reported-by: Puja Gupta <pmgupta.akl@gmail.com>
Signed-off-by: Anup Patel <anup@brainfault.org>