scripts/populate: cleanups and misc fixes

- it's a POSIX compliant shell script: drop bash, use /bin/sh
- fix help text
- use an absolute path for sysroot
- replace "echo" with "printf"
- replace "stat -c '%i'" with "ls -1id"
- replace "pushd / popd" with "cd / cd -"
- remove superfluous break
- bail out if required lib not found, except if forced

 source config/global.in

 source config/target.in

 source config/toolchain.in

 source config/kernel.in

 source config/binutils.in

 source config/cc.in

 source config/libc.in

 source config/debug.in

 source config/companion_libs.in

 source config/companion_tools.in