Fix fastboot to cope with hammerhead's implicit hex.

Before:

  wiping userdata...
  Couldn't parse partition size '3321fa800'.
  wiping cache...
  Couldn't parse partition size '2bc00000'.

Groan. So much variation between bootloaders. I wish we had a reference
bootloader like ChromeOS does.

I've also removed a harmless warning:

  couldn't parse max-download-size ''

Change-Id: Ia1099d2f87000ebb96622ad9171819a1326fa249