shell: Add new way of adding subcommands

Added macro SHELL_SUBCMD_SET_CREATE which creates a set of subcommands.
SHELL_SUBCMD_ADD and SHELL_SUBCMD_COND_ADD can be used from any file to
add command to the set. This approach allows to have subcommands added
from multiple files.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>