It was documented that `lb config` would catch invalid combinations of
bootloader type and image type or filesystem, but in practice only a
small subset of the erroronous cases were caught. This makes the
checking more thorough.

It also updates the checks in the various bootloader binary scripts, to
be explicit about what image types *are* supported (raising an error on
anything else) rather than throwing an error on an list of unsupported
image types (that might become incomplete in the future).

Finally, it makes checks against *all* supplied bootloaders, rather than
just the first one as was previously done.

I tested these to work as expected (only the config, not the full
build):

	lb config -b iso --bootloaders syslinux; rm -rf config
	lb config -b iso --bootloaders syslinux,grub-efi; rm -rf config
	lb config -b iso --bootloaders grub-pc,grub-efi; rm -rf config
	lb config -b isohybrid --bootloaders syslinux; rm -rf config
	lb config -b isohybrid --bootloaders syslinux,grub-efi; rm -rf config
	lb config -b isohybrid --bootloaders grub-pc,grub-efi; rm -rf config
	lb config -b hdd --bootloaders syslinux; rm -rf config
	lb config -b hdd --bootloaders syslinux,syslinux-efi; rm -rf config
	lb config -b hdd --bootloaders syslinux-efi; rm -rf config
	lb config -b hdd --binary-filesystem ext2 --bootloaders syslinux; rm -rf config
	lb config -b hdd --binary-filesystem ext2 --bootloaders syslinux,syslinux-efi; rm -rf config
	lb config -b netboot --bootloaders syslinux; rm -rf config
	lb config -b hdd --bootloaders syslinux; rm -rf config
	lb config -b hdd --bootloaders syslinux,syslinux-efi; rm -rf config
	lb config -b hdd --bootloaders syslinux-efi; rm -rf config

And tested to fail (or warn) as expected at config time:

	lb config -b iso --bootloaders syslinux,syslinux-efi; rm -rf config
	lb config -b iso --bootloaders syslinux-efi; rm -rf config
	lb config -b isohybrid --bootloaders syslinux,syslinux-efi; rm -rf config
	lb config -b isohybrid --bootloaders syslinux-efi; rm -rf config
	lb config -b hdd --bootloaders syslinux,grub-efi; rm -rf config
	lb config -b hdd --bootloaders grub-pc,grub-efi; rm -rf config
	lb config -b hdd --binary-filesystem other_fs --bootloaders syslinux; rm -rf config
	lb config -b hdd --binary-filesystem other_fs --bootloaders syslinux,syslinux-efi; rm -rf config
	lb config -b netboot --bootloaders syslinux,grub-efi; rm -rf config
	lb config -b netboot --bootloaders syslinux,syslinux-efi; rm -rf config
	lb config -b netboot --bootloaders syslinux-efi; rm -rf config
	lb config -b netboot --bootloaders grub-pc,grub-efi; rm -rf config