FWU: Add support for `fwu_fip` target

FWU: Add support for `fwu_fip` target

Firmware update feature needs a new FIP called `fwu_fip.bin` that
includes Secure(SCP_BL2U, BL2U) and Normal world(NS_BL2U) images
along with the FWU_CERT certificate in order for NS_BL1U to load
the images and help the Firmware update process to complete.

This patch adds the capability to support the new target `fwu_fip`
which includes above mentioned FWU images in the make files.

The new target of `fwu_fip` and its dependencies are included for
compilation only when `TRUSTED_BOARD_BOOT` is defined.

Change-Id: Ie780e3aac6cbd0edfaff3f9af96a2332bd69edbc