config_distro_bootcmd: do not scan boot partitions if distro_bootpart is defined
For some projects, designing the active bootable partition is done by
setting distro_bootpart in the environment instead of changing the bootable
flag on the active partition.
This patch makes it possible to support both models by scanning boot
partitions only if distro_bootpart is not defined in the environment.
Signed-off-by: Hugo Villeneuve <hvilleneuve@dimonoff.com>