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>

tools/buildman/README

@@ -670,7 +670,9 @@ snapper9g45=${at91-boards} BUILD_TAG=443
 This will use 'make ENABLE_AT91_TEST=1 BUILD_TAG=442' for snapper9260
 and 'make ENABLE_AT91_TEST=1 BUILD_TAG=443' for snapper9g45. A special
 variable ${target} is available to access the target name (snapper9260 and
-snapper9g20 in this case). Variables are resolved recursively.
+snapper9g20 in this case). Variables are resolved recursively. Note that
+variables can only contain the characters A-Z, a-z, 0-9, hyphen (-) and
+underscore (_).
 
 It is expected that any variables added are dealt with in U-Boot's
 config.mk file and documented in the README.

tools/buildman/toolchain.py

@@ -198,7 +198,7 @@ class Toolchains:
         >>> 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)