fastboot: Introduce FASTBOOT_DEVICE alias for ANDROID_SERIAL environment variable

The ANDROID_SERIAL env is useful for managing multiple
directly connected devices at one time.

Unfortunately for network connected devices, its harder to use.

This is because both adb and fastboot use the same ANDROID_SERIAL
environment value to identify the device to use, and while that
one value works for directly connected devices, when using
network connected devices, the fastboot and adb port numbers may
differ for the same device.

So if I set: `ANDROID_SERIAL="tcp:127.0.0.1:44403"`
fastboot will work, but when I type `adb shell`, I'll get:
 adb: device 'tcp:127.0.0.1:44403' not found

As `adb devices` outputs:
List of devices attached
localhost:36697 device

To resolve this, we need separate environment variables,
so introduce a FASTBOOT_DEVICE variable for fastboot.

If FASTBOOT_DEVICE is set, it will use that, and if not
it will fall back to the ANDROID_SERIAL if it is set.

Using an explicit -s argument will still override both.

Change-Id: Icd7db6f29e51ed2decd219e35537f6ed98d698b8
Signed-off-by: John Stultz <jstultz@google.com>