Makefile: Change CONFIG_SPL_PAD_TO to image offset

Change CONFIG_SPL_PAD_TO from a link address to an image offset since this is
more handy and closer to the purpose of this config.

Automatically define CONFIG_SPL_PAD_TO to CONFIG_SPL_MAX_SIZE (or 0 without
CONFIG_SPL_MAX_SIZE).

Test that CONFIG_SPL_PAD_TO >= CONFIG_SPL_MAX_SIZE if CONFIG_SPL_PAD_TO is
non-zero.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>