eficonfig: EFI_VARIABLE_APPEND_WRITE is not set for null key

The signed null key with authenticated header is used to clear
the PK, KEK, db and dbx. When CONFIG_EFI_MM_COMM_TEE is enabled
(StMM and OP-TEE based RPMB storage is used as the EFI variable
storage), clearing KEK, db and dbx by enrolling a signed null
key does not work as expected if EFI_VARIABLE_APPEND_WRITE
attritube is set.

This commit checks the selected file is null key, then
EFI_VARIABLE_APPEND_WRITE attibute will not be used for the null key.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>