manifold supporting and using ISOLINUX now

manifold supporting and using ISOLINUX now

The corresponding commit 1004CF6A89472F8D29F from MirBSD touching
https://www.mirbsd.org/cvs.cgi/src/sys/arch/i386/stand/bootxx/bootxx.S
introduced support for loading ISOLINUX (with similar hacks as the
isohybrid loader uses since SYSLINUX lacks a stable API).

This commit updates bootgrub.mksh from bootxx.S (and an up-to-date
mkbxinst.sh) and introduces bootilnx.mksh (which is built the same
but with BOOT_TYPE=ilnx on the make command line) to match current
MirBSD sources (and OpenADK, which uses bootgrub.mksh now also).

Furthermore, the default for HYBRID_METHOD is still 'manifold' but
installs ISOLINUX now; the new value 'grub2' asks for the current
behaviour. Note that the templates/boot/grub/ directory (with both
GRUB 2 and GRUB-legacy) is still necessary because they’re addons,
and useful in system rescue and booting purposes.

Signed-off-by: Thorsten Glaser <tg@mirbsd.org>