oe-selftest: add wic.Wic.test_grub_install_pcbios

oe-selftest: add wic.Wic.test_grub_install_pcbios

wic.Wic.test_grub_install_pcbios test

Test updates to the bootimg_pcbios plugin that
enables support for installing grub directly
to the resulting wic image.

The test checks to see if the wics plugin
generates a wic image. Then see's if normal.mod
and grub.cfg are located in the boot partition.

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