wic: Prepare wicboot to allow custom bootloader config
Currently wic does the bootloader configuration file on the fly.
This change introduce a configfile variable for the bootloader;
this is to have a user defined configuration file for the
bootloaders (grub, syslinux, and gummiboot). This is particular
useful when having a multiboot system or scripts embedded in the
configuration file.
[YOCTO #8728]
Signed-off-by: Mariano Lopez <mariano.lopez@linux.intel.com> Signed-off-by: Ross Burton <ross.burton@intel.com>
