fastboot: support for block write and boot partition access

flash now supports writing by block numbers instead of partition names.
The following command writes an image to block 0x42
$ fastboot flash 0x42 u-boot.img

emmc boot partition can be unlocked for access via standard fastboot

fastboot flashing unlock_critical
fastboot flashing lock_critical

See eMMC spec for details of boot_partition_enable, CSD register byte 179

to program a bootloader, such as fastboot
$ fastboot flashing unlock_critical
$ fastboot flash 0x42 u-boot.img
$ fastboot flashing lock_critical

This also supports raw writes to environment configuration.  To
invalidate the saved uboot environment (and effectively reset it to default)

dd if=/dev/zero of=zero.bin bs=512 count=1
$ fastboot flashing unlock_critical
$ fastboot flash 0x1ff0 zero.bin
$ fastboot flashing lock_critical

Signed-off-by: Pat Kusbel <pat@kusbel.com>