modified asset files not rebuilt into .rp6502 files

[patrick-flynn]

Changes made to binary assets (e.g., asset.bin) don't seem to trigger regeneration of the corresponding asset.bin.rp6502 file that is then bundled into the executable sent to the system to run.

Reproduce (Linux):

1. Clone the "examples" repo
2. build and run the altair demo, see the vintage picture.
3. go to the src directory
4. randomize the altair.dat.bin file, e.g. dd if=/dev/random of=altair.dat.bin bs=57600 count=1
5. build and run the altair demo, see the same vintage picture.
6. Delete Cmake Cache and reconfigure
7. build the altair demo, see the randomized image.