unzip: Use the "-q" option to silence unzipping of source files

Add and use the "UNZIP" variable instead of calling directly unzip
because the variable contains the "-q" option to silence "unzip" so it
doesn't show the list of files extracted just like when tar files are
being unpacked.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>