Fix "fastboot flashing".

...without breaking "fastboot oem". Turns out the bootloader actually needs
to know whether it's getting a "flashing" or "oem" command.

Bug: http://b/38321935
Test: "fastboot flashing unlock" "fastboot oem uart enable"
Change-Id: I2b3c0b42b01e3c884aa794fa273034a366985089