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

 syntax: glob

 # Generated files

 Makefile

 ct-ng

 docs/ct-ng.1

 docs/ct-ng.1.gz

 paths.mk

 scripts/crosstool-NG.sh

 scripts/saveSample.sh

 scripts/showTuple.sh

 # Temporaries

 .*.swp

 log.*

 # Working files

 kconfig/conf

 kconfig/mconf

 kconfig/**.o

 kconfig/**.dep

 config.gen/

 .config

 .config.old

 targets/