eficonfig: carve out efi_get_next_variable_name_int calls

To retrieve the EFI variable name by efi_get_next_variable_name_int(),
the sequence of alloc -> efi_get_next_variable_name_int ->
realloc -> efi_get_next_variable_name_int is required.
In current code, this sequence repeatedly appears in
the several functions. It should be curved out a common function.

This commit also fixes the missing free() of var_name16
in eficonfig_delete_invalid_boot_option().

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/include/efi_loader.h b/include/efi_loader.h
index 0899e29..6991768 100644
--- a/include/efi_loader.h
+++ b/include/efi_loader.h
@@ -708,6 +708,8 @@
 int efi_link_dev(efi_handle_t handle, struct udevice *dev);
 int efi_unlink_dev(efi_handle_t handle);
 bool efi_varname_is_load_option(u16 *var_name16, int *index);
+efi_status_t efi_next_variable_name(efi_uintn_t *size, u16 **buf,
+				    efi_guid_t *guid);
 
 /**
  * efi_size_in_pages() - convert size in bytes to size in pages