uboot: Add the missing disk write operation support

uboot: Add the missing disk write operation support

uboot_disk_write() is currently lacking the write support
to storage devices because, historically, those devices did not
implement block_write() in U-Boot.

The solution has been tested using a patched U-Boot loading
and booting GRUB in a QEMU vexpress-a9 environment.
The disk write operations were triggered with GRUB's save_env
command.

Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>