xprintk: Subsystem-specific printk-like functions

Signed-off-by: Tal Shorer <tal.shorer@gmail.com>
added support for building and uploading modules for a buildroot-made vm.
extra environment variables such as CROSS_COMPILE and ARCH are required when compiling for the vm.

Signed-off-by: Tal Shorer <tal.shorer@gmail.com>
Merge branch 'master' into usblb