0.2.3: efi: Implement writing to block device

Using the SectorWrite trait added to the block device implement writing
to the block device. This is useful for bootloaders that need to write
to the block device in order to be able support fallback booting.

In particular this enables the firmware to be used with Ubuntu 19.10
("eoan".)

Signed-off-by: Rob Bradford <robert.bradford@intel.com>