cypress: Build firmware.c the same way as other files
cypress: Build firmware.c the same way as other files

We were building firmware.c as a dependency of the link stage, instead
of including in the list of C files. I can't see a reason not to have
the rule generated the same as the other C files, so do it this way.

Signed-off-by: Joel Stanley <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@jms.id.au>

rohit91/HDMI2USB,mithro/HDMI2USB,mithro/HDMI2USB,timvideos/HDMI2USB-jahanzeb-firmware,mithro/HDMI2USB,timvideos/HDMI2USB-jahanzeb-firmware,rohit91/HDMI2USB,rohit91/HDMI2USB,mithro/HDMI2USB,rohit91/HDMI2USB,rohit91/HDMI2USB,mithro/HDMI2USB,timvideos/HDMI2USB-jahanzeb-firmware,timvideos/HDMI2USB-jahanzeb-firmware,timvideos/HDMI2USB-jahanzeb-firmware