New module for android bootimg filesystem

[Shea Levy]

Hello all,

On the android-bootimg branch of my github fork [1], I've implemented 
initial support for the android bootimg format[2]. Currently the kernel 
and ramdisk are accessible on the filesystem; future work may include 
exporting the kernel command line, expected memory offsets, etc. somehow 
(perhaps in a file that sets some grub variables that is meant to be 
source'd). The developer's manual doesn't mention how to submit changes 
using git; should I squash this all down into one commit, send a patch 
per commit, or something else?

Thanks,
Shea

[1]: https://github.com/shlevy/grub/tree/android-bootimg
[2]: 
https://android.googlesource.com/platform/system/core/+/506d233e7ac8ca4efa80768153d842c296477f99/mkbootimg/bootimg.h