devtool: upgrade: handle recipes that use named SRC_URI checksums

devtool: upgrade: handle recipes that use named SRC_URI checksums

devtool upgrade did not properly handle setting SRC_URI checksums for
recipes that use named SRC_URI entries and also use those names in the
SRC_URI checksums. A further complication was where the name contained
an expression that changed with the version e.g. ${PV} (probably quite
rare, but the dnsmasq recipe in meta-networking is currently one such
recipe.) All of these are now handled properly.

Additionally, drop the _get_checksums() function that wasn't being
called from anywhere in the code.

Note that this now turns nowrap_vars in recipeutils.py to be a list of
regexes, hence things such as [ and ] need to be appropriately escaped.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>