Don't set boot and legacy_boot flags on GPT PReP

Setting boot flag on GPT PReP resets prep flag which leads to
grub2-install unable to install a bootloader (bnc#880094)

Signed-off-by: Dinar Valeev <dvaleev@suse.com>
Merge pull request #101 from k0da/master

Don't set boot and legacy_boot flags on GPT PReP