py.mk: allow translation to be overriden in GNUmakefile

I like to use local makefile overrides, in the file GNUmakefile
(or, on case-sensitive systems, makefile) to set compilation choices.
However, writing
    TRANSLATION := de_DE
    include Makefile
did not work, because py.mk would override the TRANSLATION := specified
in an earlier part of the makefiles (but not from the commandline).

By using ?= instead of := the local makefile override works, but when
TRANSLATION is not specified it continues to work as before.

Author: jepler

py/py.mk

@@ -7,7 +7,7 @@ HEADER_BUILD = $(BUILD)/genhdr
 # file containing qstr defs for the core Python bit
 PY_QSTR_DEFS = $(PY_SRC)/qstrdefs.h
 
-TRANSLATION := en_US
+TRANSLATION ?= en_US
 
 # If qstr autogeneration is not disabled we specify the output header
 # for all collected qstrings.