build: Name firmware bin and elf file by the name of the firmware

Having different file names makes it easier to differentiate
the firmware binaries. It also removes the need for renaming
them when they are placed in a common directory.

The firmware.bin binary file is still generated by the build
system. A subsequent patch will remove its generation giving
time to remove any dependency on it.

Change-Id: Id38c20d667f802f18abf090b0c331b796c2ce23d
Signed-off-by: Ronald Cron <ronald.cron@arm.com>