src/Makefile: make 'make clean' clean all produced files

[ia]

TL; DR: make make clean clean all produced files.

Steps to reproduce:

• run build: $ make
• clean produced files: $ make clean

What should happen:

• usually, it's a common practice when after make clean all produced files are deleted

What actually happens:

• src/common/hydrafw_version.hdr kept as "leftover"

At first I was thinking about adding a separate clean target into Makefile but since internal variables of rules.mk from ChibiOS support "custom enhancements", just adding command rm -f src/common/hydrafw_version.hdr to special variable do the trick.

I did successfully test it myself on Ubuntu 18.04. But I did not test it on Windows/mingw unfortunately.

P.S. Happy New Year! :)

[bvernoux]

Thanks for your contribution