luci-base: add hexstring datatype

Add datatype 'hexstring' for input validaiton datatypes.

It will accept any hexadecimal string.

(no length validation, as rangelength can be used for that.)

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
(cherry picked from commit 11f23559e4)

modules/luci-base/luasrc/cbi/datatypes.lua

@@ -267,6 +267,13 @@ function wepkey(val)
 	end
 end
 
+function hexstring(val)
+        if val then
+                return (val:match("^[a-fA-F0-9]+$") ~= nil)
+        end
+        return false
+end
+
 function string(val)
 	return true		-- Everything qualifies as valid string
 end