binman should create a list of output files

Hi Simon,

During the discussion on [1] I had the idea if binman could create a list of all output files, so that it is possible to do a real clean in the u-boot makefile.

Currently there are not all binaries listed in the files list and especially during the port of some boards to binman we had some trouble due to fragments of old/previous builds.

What do you think?

[1] https://lists.denx.de/pipermail/u-boot/2021-August/458530.html