bitbake: parse: Forbid ambiguous assignments to ${.}, ${+}, and ${:} variables

Old code that parse variable names in assignment commands behave differently for
variables that ends with special symbol for single-character variable names and
multi-character variable names. For example:

  A+="1"   # Change variable ${A}, '+' glued to '='
  A+ = "1" # Change variable ${A+}

  +="1"    # Change variable ${+}, the '+' symbol not part of assignment operator
  + = "1"  # Change variable ${+}

New code would always assume that '.=', '+=', and ':=' is assignment operator.
As result code like the following would raise parsing error

  +="value"

While code with extra spaces would work as before

  + = "value" # Change variable ${+}

This change allow to catch issues in code that generate bitbake configuration
files in a manner like "echo ${VARNAME}+=${VALUE} >> conf/local.conf"

(Bitbake rev: 93059aad13a12cd69d86368795c88e5349197d5d)

Signed-off-by: Nikolai Merinov <n.merinov@inango-systems.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/parse/parse_py/ConfHandler.py

@@ -20,7 +20,7 @@ from bb.parse import ParseError, resolve_file, ast, logger, handle
 __config_regexp__  = re.compile( r"""
     ^
     (?P<exp>export\s+)?
-    (?P<var>[a-zA-Z0-9\-_+.${}/~:]+?)
+    (?P<var>[a-zA-Z0-9\-_+.${}/~:]*?)
     (\[(?P<flag>[a-zA-Z0-9\-_+.][a-zA-Z0-9\-_+.@/]*)\])?
 
     \s* (
@@ -166,6 +166,8 @@ def feeder(lineno, s, fn, statements, baseconfig=False, conffile=True):
     m = __config_regexp__.match(s)
     if m:
         groupd = m.groupdict()
+        if groupd['var'] == "":
+            raise ParseError("Empty variable name in assignment: '%s'" % s, fn, lineno);
         ast.handleData(statements, fn, lineno, groupd)
         return