grml2iso: switch from isohybrid to xorriso/isohybrid to properly support EFI boot

grml2iso: switch from isohybrid to xorriso/isohybrid to properly support EFI boot

This is the related change for grml-live's commit 6725bfa1ffc4, where we
introduced EFI support on 32bit systems and switched from isohybrid to
xorriso/isohybrid combination.

The isohybrid binary doesn't support 32-bit FI systems and fails hard
when using `--uefi` on a 32-bit ISO.  But xorriso with appropriate
options for EFI usage (see $BOOT_ARGS) and
/usr/lib/ISOLINUX/isohdpfx.bin from the isolinux package seems to
provide everything we need.

JFTR: To properly support 64bit EFI boot with our grml96 ISOs, we also
had to adjust the order of the provided ISOs in the grml2iso command
line in our release process.

Closes: grml/grml#149