CLSA-2022-1669388927 grub2: Fix of 7 CVEs

CVE-2021-3981: Fix default privileges of grub.cfg file - CVE-2022-28736: Fix use-after-free bug when grubcmdchainloader is executed more than once before a boot attempt is performed. - CVE-2021-3695: Drop greyscale support to fix heap out-of-bounds write - CVE-2021-3696: Fix out of range...