grub: add --target=i386-pc for bios systems

From Arch wiki:
--target=i386-pc instructs grub-install to install for BIOS systems only. It is recommended
to always use this option to remove ambiguity in grub-install.
Merge pull request #181 from codeworkx/master

grub: add --target=i386-pc for BIOS systems
Merge pull request #58 from calamares/master

Merge master