grub2: bump to 2.06-rc1

grub2: bump to 2.06-rc1

When building GRUB with binutils 2.35.2 or later, an error occurs due to
a section .note.gnu.property that is placed at an offset such that
objcopy needs to pad the img file with zeros. This in turn causes the
following error: "error: Decompressor is too big.".

The fix accepted by upstream patches a python script that isn't executed
at all when building GRUB with OpenWrt buildroot. There's another patch
that patches the files generated by that python script directly, but by
including it we would deviate further from upstream. Instead of doing
that, simply bump to the latest release candidate.

As one of the fixes for the CVEs causes grub to crash on some x86
hardware using legacy BIOS when compiled with -O2, filter -O2 and
-O3 out of TARGET_CFLAGS.

Fixes the following CVEs:
- CVE-2020-14372
- CVE-2020-25632
- CVE-2020-25647
- CVE-2020-27749
- CVE-2020-27779
- CVE-2021-3418
- CVE-2021-20225
- CVE-2021-20233

Runtime-tested on x86/64.

Fixes: FS#3790
Suggested-by: Dirk Neukirchen <plntyk.lede@plntyk.name>
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Acked-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>