tools: memory-hotplug fix unexpected operator error

on-off-test uses "$UID != 0" to test for root, but $UID is a construct
specific to bash.  Using /bin/sh that isn't bash results in the
following error (due to the "$UID" part expanding to nothing):

  ./on-off-test.sh: 9: [: !=: unexpected operator

Change Makefile to use bash instead.

Signed-off-by: Shuah Khan <shuah.kh@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>