buildman: Allow make-flags variables to include '-' and '_'

These characters are commonly used in variables, so permit them. Also
document the permitted characters.

Reported-by: Tom Rini <trini@ti.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/buildman/toolchain.py b/tools/buildman/toolchain.py
index 1b9771f..0e91294 100644
--- a/tools/buildman/toolchain.py
+++ b/tools/buildman/toolchain.py
@@ -198,7 +198,7 @@
         >>> tcs.ResolveReferences(var_dict, 'this=${oblique}_set${first}nd')
         'this=OBLIQUE_setfi2ndrstnd'
         """
-        re_var = re.compile('(\$\{[a-z0-9A-Z]{1,}\})')
+        re_var = re.compile('(\$\{[-_a-z0-9A-Z]{1,}\})')
 
         while True:
             m = re_var.search(args)