Add support for init mknod command.

This is required for statically allocated devices.
These don't hit ueventd and hence doesn't automatically get a device node.

Change-Id: I83d19f0666e1f1bbb369b226fba019994d403a89
init: Implement exec support

Code copied from CM7 implementation by Steve Kondik

Change-Id: Ie8f11e9dc9e552aa7a4fe354995daaf8f0d99a11