feat: move el3_payload to the build directory

Similar to tf-a's fiptool, el3_payload should be built in the build
directory so that many tftf builds can run at the same time. Currently,
only the output is stashed, however that still produces conflicts. This in
turns allows for running many CI jobs in parallel.

Change-Id: Ice73ec96c6a9497684408e2c01ceee2f99479ea9
Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
2 files changed