selftest/wic.py: Add test for custom bootloader config
This change just add anoher test to the wic module.
It will try to create a image with a custom bootloader
configuration. This test will use the example image
directdisk-bootloader-config to test the configfile
option for the bootloaders.
[YOCTO #8728]
Signed-off-by: Mariano Lopez <mariano.lopez@linux.intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>