Trouble w/ system-as-root setups where boot.img has no initrd

[johnstultz-work]

With newer AOSP environments, the system partition is used as rootfs, and there is no initrd in the boot.img.

Unfortunately abootimg -x seems to fail in this case:
/home/jstultz/noinitrd-boot.img: ramdisk size is null
/home/jstultz/noinitrd-boot.img: not a valid Android Boot Image.

Probably need to adjust the code to not make a null ramdisk size a fatal error.

[johnstultz-work]

Created a pullrequest here to try to resolve this:
#3