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/pxe_utils.c b/cmd/pxe_utils.c
index 9a30629..067c24e 100644
--- a/cmd/pxe_utils.c
+++ b/cmd/pxe_utils.c
@@ -351,7 +351,7 @@
 	int err;
 
 	/* Get the main fdt and map it */
-	fdt_addr = simple_strtoul(env_get("fdt_addr_r"), NULL, 16);
+	fdt_addr = hextoul(env_get("fdt_addr_r"), NULL);
 	working_fdt = map_sysmem(fdt_addr, 0);
 	err = fdt_check_header(working_fdt);
 	if (err)
@@ -364,7 +364,7 @@
 		return;
 	}
 
-	fdtoverlay_addr = simple_strtoul(fdtoverlay_addr_env, NULL, 16);
+	fdtoverlay_addr = hextoul(fdtoverlay_addr_env, NULL);
 
 	/* Cycle over the overlay files and apply them in order */
 	do {