Add emmc_size envvar accessible from fastboot.

There are boards with several different eMMC sizes. We can easily
build gpt for each size and then flash the correct one to the
board. In order to identify this size, we can run fastboot command:

> fastboot getvar mmc_size

Change-Id: I32dac02b974753ed59f6db0bfbc5f7289f391f4c
1 file changed