Make efiSystemPartition mount point configurable in partition.conf.

Copy the setting into the global storage. Make the EraseDiskPage in the
partition module and the grub and bootloader modules read it from there.
Do not hardcode /boot as the path anymore.

I set the default path to /boot/efi because I think that's the most
common setting. At least Fedora and Debian use that path. But we can
change the default setting if you think I'm wrong, as long as it remains
configurable.

There is no separate setting for efi_directory_firmware anymore. The EFI
firmwares expect that directory to always be in the "EFI" path in the
EFI System Partition. Distributions using /boot/efi actually have that
directory under /boot/efi/EFI.