utils/checkpackagelib/lib_mk.py: fix check for overridden variable

Currently this .mk snippet results in unexpected behavior from
check-package:
|VAR_1 = VALUE1
|ifeq (condition)
|VAR_1 := $(VAR_1), VALUE2
|endif

Fix commit "163f160a8e utils/{check-package, checkpackagelib}:
consistently use raw strings for re.compile" that ended up doing this:
-    CONCATENATING = re.compile("^([A-Z0-9_]+)\s*(\+|:|)=\s*\$\(\\1\)")
+    CONCATENATING = re.compile(r"^([A-Z0-9_]+)\s*(\+|:|)=\s*\$\(\\1\)")

But raw strings do not expect escaping when referencing \1 and the
pattern ends up searching for a raw '\\1' instead of an occurrence of
the first pattern inside parenthesis.

|$ python3
|Python 3.8.10 (default, Sep 28 2021, 16:10:42)
|[GCC 9.3.0] on linux
|Type "help", "copyright", "credits" or "license" for more information.
|>>> import re
|>>> p1 = re.compile('(foo)bar\\1')
|>>> p2 = re.compile(r'(foo)bar\\1')
|>>> p3 = re.compile(r'(foo)bar\1')
|>>> s1 = 'foobarfoo'
|>>> s2 = 'foobar\\1'
|>>> print(p1.search(s1))
|<re.Match object; span=(0, 9), match='foobarfoo'>
|>>> print(p2.search(s1))
|None
|>>> print(p3.search(s1))
|<re.Match object; span=(0, 9), match='foobarfoo'>
|>>> print(p1.search(s2))
|None
|>>> print(p2.search(s2))
|<re.Match object; span=(0, 8), match='foobar\\1'>
|>>> print(p3.search(s2))
|None
|>>>

So use '\1' instead of '\\1' in the raw string.

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Titouan Christophe <titouan.christophe@railnova.eu>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
(cherry picked from commit 5bbedea9c2317bfb04d1efd5fb157e725e86c2fb)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

utils/checkpackagelib/lib_mk.py

@@ -77,7 +77,7 @@ class Indent(_CheckFunction):
 
 
 class OverriddenVariable(_CheckFunction):
-    CONCATENATING = re.compile(r"^([A-Z0-9_]+)\s*(\+|:|)=\s*\$\(\\1\)")
+    CONCATENATING = re.compile(r"^([A-Z0-9_]+)\s*(\+|:|)=\s*\$\(\1\)")
     END_CONDITIONAL = re.compile(r"^\s*({})".format("|".join(end_conditional)))
     OVERRIDING_ASSIGNMENTS = [':=', "="]
     START_CONDITIONAL = re.compile(r"^\s*({})".format("|".join(start_conditional)))