efi: Move a few helper functions into the common efi/ dir

Checking if a variable is a load option is useful for the app, so move
efi_varname_is_load_option() and u16_tohex() into a new helper.c file.

Signed-off-by: Simon Glass <sjg@chromium.org>

lib/efi/Makefile

@@ -5,5 +5,6 @@
 
 obj-y += basename.o
 obj-y += device_path.o
+obj-y += helper.o
 obj-y += memory.o
 obj-y += run.o

lib/efi/helper.c

@@ -0,0 +1,42 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Copyright (c) 2020, Linaro Limited
+ */
+
+#define LOG_CATEGORY LOGC_EFI
+
+#include <string.h>
+#include <linux/types.h>
+
+static int u16_tohex(u16 c)
+{
+	if (c >= '0' && c <= '9')
+		return c - '0';
+	if (c >= 'A' && c <= 'F')
+		return c - 'A' + 10;
+
+	/* not hexadecimal */
+	return -1;
+}
+
+bool efi_varname_is_load_option(u16 *var_name16, int *index)
+{
+	int id, i, digit;
+
+	if (memcmp(var_name16, u"Boot", 8))
+		return false;
+
+	for (id = 0, i = 0; i < 4; i++) {
+		digit = u16_tohex(var_name16[4 + i]);
+		if (digit < 0)
+			break;
+		id = (id << 4) + digit;
+	}
+	if (i == 4 && !var_name16[8]) {
+		if (index)
+			*index = id;
+		return true;
+	}
+
+	return false;
+}