makefile: better coverage

Added precoverage step to ensure the percentage of lines covered are accurate

Added remove feature to better target files

Fixed coverage routine at the end

Removed make check in precoverage

Excluded /bin/* files

fixed output file

Change-Id: I7a4a4947d7a5491543e58b1c4b029db3cbcd0b7c
Reviewed-by: Sébastien Blin <sebastien.blin@savoirfairelinux.com>