Add || exit 1 to all critical build steps (mkdir, cd, touch, configure,
make, grub-mkimage) so failures surface immediately instead of silently
continuing. Verify gcc/make/bison/flex are present before starting the
build. Update grub.cfg help text to reference fat_tool.py instead of
unavailable mtools.
Builds GRUB 2.12 for the host machine (not Redox target) using template=custom.
Produces a 540 KiB standalone PE32+ EFI binary with GPT, FAT, ext2,
chainloader, and utility modules. The grub.cfg chainloads the Redox
bootloader from EFI/REDBEAR/redbear.efi.
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
vasilito