meta: Fix Deprecated warnings from regexs

meta: Fix Deprecated warnings from regexs

Fix handling of escape characters in regexs and hence fix python
Deprecation warnings which will be problematic in python 3.8.

Note that some show up as:

"""
meta/classes/package.bbclass:1293: DeprecationWarning: invalid escape sequence \.  

"""

where the problem isn't on 1293 in package.bbclass but in some _prepend to a
package.bbclass function in a different file like mesa.inc, often from
do_package_split() calls.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>