bitbake: lib/bb/utils: add safeguard against recursively deleting things we shouldn't

bitbake: lib/bb/utils: add safeguard against recursively deleting things we shouldn't

Add some very basic safeguard against recursively deleting paths such
as / and /home in the event of bugs or user mistakes.

Addresses [YOCTO #7620].

(Bitbake master rev: 56cddeb9e1e4d249f84ccd6ef65db245636e38ea)

(Bitbake rev: fbf1c39641f78d553961974a2bb96256eb9496e7)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>