efi_loader: variable: support APPEND_WRITE

If EFI_VARIABLE_APPEND_WRITE is specified in attributes at
efi_set_variable(), specified data will be appended to the variable's
original value. Attributes other than APPEND_WRITE should not be
modified.

With this patch, APPEND_WRITE test in 'variables' selftest will pass.

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