lib/oe/recipeutils: patch_recipe(): fix replacing varflag values

lib/oe/recipeutils: patch_recipe(): fix replacing varflag values

The code here wasn't correctly getting the variable history for
varflags, so for example if you did a devtool upgrade on a recipe where
the SRC_URI checksums were in the .inc file (typical for python recipes
in order to support both python 2 and 3) then after the upgrade the
new values would be set in the recipe and the old values were left in
the .inc, which is not right. Teach the code here how to get the history
for varflags so it works properly.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>