efi_loader: variable: return error for APPEND_WRITE

The current efi_st_variable() doesn't support EFI_VARIABLE_APPEND_WRITE
attiribute for now, and so should return an error.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>

Fix typos is commit message.
Add TODO comment.

Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>