grub.cfg: remove duplicate setting of gfx*
make_iso_img: update MBR patching for latest grub2

Lastest GRUB2 snapshots moved the core image to offset 0x800 (was
0x200). Try to find the right offset.
make_vbox_img: make it work for 64-bit images too
Remove netboot images in 'make clean'

Otherwise outdated images (built with older GRUB)  could be included
in the build.
grub.cfg: drop pldnr_menuentry debug message
Load GRUB 'progress' module if available

Use the GRUB 'progress' module to show progress of loading boot files.
Do not wait for boot media on network boot
Better fix for finding ISO image by UUID

It failed, because udev found the ISO UUID both on the whole
image and on the 'Gap2' partition made by xorrriso. The by-uuid
symlink (used by mount and blkid) would point to the false-match.

Fixing this by lowering 'link_priority' of the 'Gap' partitions.