fiptool: Support non-decimal --align arguments

An alignment value of 0x4000 is much easier to type than 16384,
so enhance get_image_align() to recognize a 0x prefix for hexadecimals.

Signed-off-by: Andreas Färber <afaerber@suse.de>
diff --git a/tools/fiptool/fiptool.c b/tools/fiptool/fiptool.c
index 542a946..0503696 100644
--- a/tools/fiptool/fiptool.c
+++ b/tools/fiptool/fiptool.c
@@ -646,7 +646,7 @@
 	unsigned long align;
 
 	errno = 0;
-	align = strtoul(arg, &endptr, 10);
+	align = strtoul(arg, &endptr, 0);
 	if (*endptr != '\0' || !is_power_of_2(align) || errno != 0)
 		log_errx("Invalid alignment: %s", arg);