build-pac.sh: option to send warnings and errors to a log file

this will copy warnings and errors to $BUILD_TOP/warn.log, while also showing them in terminal
the log file will be overwritten every time this is invoked

Change-Id: Ibcd7ca7e053d79417590b0f404cb582d32b7b7fb
Merge "build-pac.sh: option to send warnings and errors to a log file" into pac-4.4