fastboot: Only call the bootm command if it is enabled

This fixes an error with trying to link against do_bootm() when
CONFIG_CMD_BOOTM is disabled.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>