trusted-firmware-a: Install .elf file from subdirectory

The ELF files produced are in a subdirectory named by the build target
(e.g. "bl31/bl31.elf") instead of the BUILD_PLAT directory.

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Reviewed-by: Diego Sueiro <diego.sueiro@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>