cmd: bcb: Extract '__bcb_set' from 'do_bcb_set' for internal needs

Enriching the functionality of U-Boot 'bcb' may assume using the
existing sub-commands as building blocks for the next ones.

A clean way to achive the above is to expose a number of static
routines, each mapped to an existing user command (e.g. load/set/store),
with a user/caller-friendly prototype (i.e. do not force the caller
to wrap an integer into a string).

This second patch makes '__bcb_set' available for internal needs.

No functional change intended.

Signed-off-by: Eugeniu Rosca <erosca@de.adit-jv.com>
Signed-off-by: Roman Kovalivskyi <roman.kovalivskyi@globallogic.com>