Revert "mkimage: fit: Do not tail-pad fitImage with external data"

This has been reported to break booting of U-Boot from SPL on a number
of platforms due to a lack of alignment of the external data.  The
issues this commit is addressing will need to be resolved another way.

Re-introduce a data leak in the padding for now.

This reverts commit 20a154f9.

Reported-by: Alex Kiernan <alex.kiernan@gmail.com>
Reported-by: Michael Walle <michael@walle.cc>
Tested-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Tom Rini <trini@konsulko.com>