build-target-bins: pass tftf.bin as BL33 image

In case of a tftf test case build, neither u-boot nor uefi gets built
and tftf.bin should be used as the BL33 image in FIP. So allow for
tftf.bin to be packaged in fip for tftf test case build.

Change-Id: I1f9b1a44349305c89ce2d308574b6647be707d7c
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>