bootloaders: replace use of vga=normal with vga=788 in live menu entries
For consistency with install entries (both in live-build and official Debian install discs).
Comparing with live-build created installer entries, grub-legacy and grub2 both favour vga-788 for GUI entries and vga=normal for test entries, whilst syslinux uses vga-788 for everything.
Gbp-Dch: Short