build-target-bins.sh: fix invalid indirect expansion error for uefi_out

Add a workaround for the invalid indirect expansion error encountered
when the software stack is built with $UEFI_BUILD_ENABLED set to
zero(eg: tftf) on a Ubuntu 20.04 machine. The error occurs when the if
condition checks the value assigned to local variable $uefi_out.

Signed-off-by: Shriram K <shriram.k@arm.com>
Change-Id: Ie8ec7c3332597175011048c2ba7a03722869434a