kbuild: Remove uncmd_spl logic

At this point in the conversion there should be no need to have logic to
disable some symbol during the SPL build as all symbols should have an
SPL counterpart.

The main real changes done here are that we now must make proper use of
CONFIG_IS_ENABLED(DM_SERIAL) rather than many of the odd tricks we
developed prior to CONFIG_IS_ENABLED() being available.

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