The entries for libhw* are no longer needed in .gitignore.

There is also no longer a difference between common-obj-y and
hw-obj-y, so one of those two macros is sufficient.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>