ARM: keystone2: Split monitor code / command code

When we switch to including all linker lists in SPL it is important
to not include commands as that may lead to link errors due to other
things we have already discarded.  In this case, we split the code for
supporting the monitor out from the code for loading it.

Cc: Vitaly Andrianov <vitalya@ti.com>
Cc: Nishanth Menon <nm@ti.com>
Cc: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tom Rini <trini@konsulko.com>