ddimage: Support Mac OS

ddimage: Support Mac OS

Update the ddimage script to allow it to work on Mac OS too. The biggest
difference is sysfs vs diskutil and in the syntax of the stat command
between Mac OS and Linux, unfortunately. Workarounds using ls, cut, and
columns got really fragile really quickly. Relying on stat and switching
on uname seemed the more robust solution.

(From OE-Core rev: 8962fe11a0697348affb8a1ab95abca4995470a6)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>