ipxe relies on () not being (void), so just force gnu17 for now for compat
w/ compilers that default to gnu23.
--- a/tools/firmware/etherboot/Config
+++ b/tools/firmware/etherboot/Config
@@ -7,3 +7,5 @@ CFLAGS += -DNO_POST_PROMPT
 
 CFLAGS += -UCONSOLE_SERIAL
 CFLAGS += -DCONSOLE_SERIAL=1
+
+CFLAGS += -std=gnu17