common/update: Finish Kconfig migration

We can enforce the dependencies of this module via Kconfig now, so do so
rather than with #error statements. Further, we can ensure that all
required values are set to their defaults in Kconfig, and in fact
already do so, so remove the tests here. The exception is
CONFIG_UPDATE_LOAD_ADDR which needed to be migrated to Kconfig in the
first place.

Signed-off-by: Tom Rini <trini@konsulko.com>