misc: Generalize LOAD_OFFSET support

Instead of FLASH_LOAD_OFFSET being something specific to cortex-m, add
it generally to misc/Kconfig, along with a hidden config
HAS_LOAD_OFFSET which can be selected by the architectures as they add
support for the functionality.

Change-Id: I256ff8cf4e9b8493b26354c3b93fe1f7017d4887
Signed-off-by: David Brown <david.brown@linaro.org>