ConfHandler: Improve regexp to fix mis-parsing of += and no whitespace

If you have:

FOO = "a"
FOO += "b"
FOO+= "c"

The expected result is "a b c" however we were seeing "a b" with the FOO+
variable being assigned the value "c". This isn't the expected result.

We need to make the name part of the variale non-greedy so that any + character
becomes part of the operator. This patch does that. I compared the configuration
in OE-Core before and after the change and only the test case changed.

[YOCTO #3834]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>