efi_loader: utility function to check the variable name is "Boot####"

Some commands need to enumerate the existing UEFI load
option variable("Boot####"). This commit transfers some code
from cmd/efidebug.c to lib/efi_loder/, then exposes
efi_varname_is_load_option() function to check whether
the UEFI variable name is "Boot####".

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

lib/efi_loader/efi_helper.c

@@ -190,3 +190,36 @@ int efi_unlink_dev(efi_handle_t handle)
 
 	return 0;
 }
+
+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;
+}