global: Convert simple_strtoul() with hex to hextoul()

It is a pain to have to specify the value 16 in each call. Add a new
hextoul() function and update the code to use it.

Add a proper comment to simple_strtoul() while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/bootefi.c b/cmd/bootefi.c
index cba81ff..83eab0b 100644
--- a/cmd/bootefi.c
+++ b/cmd/bootefi.c
@@ -281,7 +281,7 @@
 				return EFI_NOT_FOUND;
 			}
 		}
-		fdt_addr = simple_strtoul(fdt_opt, NULL, 16);
+		fdt_addr = hextoul(fdt_opt, NULL);
 		if (!fdt_addr) {
 			log_err("ERROR: invalid $fdt_addr or $fdtcontroladdr\n");
 			return EFI_LOAD_ERROR;
@@ -628,7 +628,7 @@
 	if (argc > 2) {
 		uintptr_t fdt_addr;
 
-		fdt_addr = simple_strtoul(argv[2], NULL, 16);
+		fdt_addr = hextoul(argv[2], NULL);
 		fdt = map_sysmem(fdt_addr, 0);
 	} else {
 		fdt = EFI_FDT_USE_INTERNAL;