bootimg_pcbios: include grub as an optional bootloader

bootimg_pcbios: include grub as an optional bootloader

Commit adds in support for installation of both
grub and syslinux using newly added grub functions.

Due to the bootimg_biosplusefi source_params['loader']
had to be named source_params['loader-bios'] so not
to create conflict in the wics plugin.

Commits also adds ability to set and or not set
source_params. If source_params set check
for both
	* syslinux
	* grub

if not set default to using syslinux as bootloader.

Signed-off-by: Vincent Davis Jr <vince@underview.tech>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>