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

 # ia64 specific config options

 # depends on EXPERIMENTAL

 config ARCH_ia64

     select ARCH_SUPPORTS_64

     select ARCH_DEFAULT_64

     select ARCH_USE_MMU

     select ARCH_SUPPORTS_BOTH_ENDIAN

     help

       The ia64 architecture, as defined by:

         http://www.intel.com/design/itanium/arch_spec.htm