scripts: remove . from $PATH

Add CT_SanitizePath function which removes entries referring to ., /tmp
and non-existing directories from $PATH, and call it early in the
build script.

If . is in PATH, gcc-4.4.4 build breaks:

[ALL ] checking what assembler to use...
/tmp/build/targets/arm-unknown-linux-uclibcgnueabi/build/gcc-core-static/arm-unknown-linux-uclibcgnueabi/bin/as
...
[ALL ] config.status: creating as

i.e. "as" is supposed to be the arm-unknown-linux-uclibcgnueabi cross assembler,
but config.status creates a local "as" script which is calling the
host assembler.

Signed-off-by: Johannes Stezenbach <js@sig21.net>
[Yann E. MORIN: style fixes + explanations]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 20dd8cef1c8adff0aa3e78ae6d7acfbc45ed5a83)

 #!/bin/sh

 canonicalizedname=$(readlink -nm "${0}")

 dirname="$(dirname "${canonicalizedname}")"

 basename="$(basename "${canonicalizedname}")"

 ld_lib_path="$(dirname "${dirname}")/lib"

 case ":${LD_LIBRARY_PATH}:" in

     *":${ld_lib_path}:"*)   ;;

     *)  LD_LIBRARY_PATH="${ld_lib_path}${LD_LIBRARY_PATH:+:${LD_LIBRARY_PATH}}";;

 esac

 export LD_LIBRARY_PATH

 exec "${dirname}/.${basename}" "$@"