Add partition support to bootloader-efi

Adds support for both GPT and MBR partitions in the UEFI bootloader. The old format, where the main filesystem was simply appended to the FAT32 boot partition, will not work. The partition scanner is somewhat simple though; it just searches for a partition with the RedoxFS UUID or the "Linux Filesystem" UUID (since I couldn't get parted to accept arbitrary UUIDs), and boots from the first one it finds.