utils/check-package: detect the use of ${} in .mk files

And warn to use $() instead.
For examples see [1] and [2].

In the regexp, search for ${VARIABLE} but:
 - ignore comments;
 - ignore variables to be expanded by the shell "$${}".

[1] http://lists.busybox.net/pipermail/buildroot/2018-July/225211.html
[2] https://github.com/buildroot/buildroot/commit/36305380db1312442623128689fe5067d9058381

Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

utils/checkpackagelib/lib_mk.py

@@ -251,3 +251,13 @@ class UselessFlag(_CheckFunction):
                     "({}#_infrastructure_for_autotools_based_packages)"
                     .format(self.filename, lineno, self.url_to_manual),
                     text]
+
+
+class VariableWithBraces(_CheckFunction):
+    VARIABLE_WITH_BRACES = re.compile(r"^[^#].*[^$]\${\w+}")
+
+    def check_line(self, lineno, text):
+        if self.VARIABLE_WITH_BRACES.match(text.rstrip()):
+            return ["{}:{}: use $() to delimit variables, not ${{}}"
+                    .format(self.filename, lineno),
+                    text]