bitbake: bitbake: ConfHandler: Allow variable flag name with a single character

Update regex pattern to allow variable flag name with a single character.

Regression tests have also been updated in `bb.parse` and
`bin/bitbake-selftest -k ParseTest` has been successfully executed.

Eliminate a trailing space as well.

(Bitbake rev: bb9e523291a3cad6e1596ee6a1e715b5e5feba8f)

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/tests/parse.py

@@ -222,6 +222,7 @@ VAR = " \\
     at_sign_in_var_flag = """
 A[flag@.service] = "nonet"
 B[flag@.target] = "ntb"
+C[f] = "flag"
 
 unset A[flag@.service]
 """
@@ -232,6 +233,7 @@ unset A[flag@.service]
         self.assertEqual(d.getVar("B"), None)
         self.assertEqual(d.getVarFlag("A","flag@.service"), None)
         self.assertEqual(d.getVarFlag("B","flag@.target"), "ntb")
+        self.assertEqual(d.getVarFlag("C","f"), "flag")
 
     def test_parse_invalid_at_sign_in_var_flag(self):
         invalid_at_sign = self.at_sign_in_var_flag.replace("B[f", "B[@f")