grub2: bump to 2.04

* GCC 8 and 9 support.
* Gnulib integration overhaul.
* RISC-V support.
* Xen PVH support.
* Native UEFI secure boot support.
* UEFI TPM driver.
* New IEEE 1275 obdisk driver.
* Btrfs RAID 5 and RIAD 6 support.
* bootin from F2FS support.
* PARTUUID support.
* VLAN support.
* Native DHCP support.
* Many ARM and ARM64 fixes.
* Many SPARC fixes.
* Many IEEE 1275 fixes.
* ...and tons of other fixes and cleanups...

Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>