Makefile: Rename ALL-y to INPUTS-y

When binman is in use, most of the targets built by the Makefile are
inputs to binman. We then need a final rule to run binman to produce the
final outputs.

Rename the variable to indicate this, and add a new 'inputs' target.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>