Replace shell build scripts with Python, clean up makefiles
* Rewrite disk/ISO image builders in pure Python (ext2, FAT16,
MBR, GPT, ISO 9660), drop dependency on dd/sfdisk/sgdisk/
mkfs/xorriso/mtools/fakeroot
* Replace ovmf-fetch.sh with ovmf_fetch.py
* Clean up all makefiles: align variables, factor common flags,
remove dead code
* UEFI loader tries multiple rootfs candidate paths
* Add bochsrc, minor rootfs config fixes