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)

Original patch from Gentoo:

gentoo/src/patchsets/binutils/2.19/08_all_binutils-RPATH_ENVVAR-smack.patch

http://sourceware.org/ml/binutils/2007-07/msg00401.html

http://sourceware.org/bugzilla/show_bug.cgi?id=4970

diff -durN binutils-2.19.orig/configure binutils-2.19/configure

--- binutils-2.19.orig/configure	2008-09-04 04:18:16.000000000 +0200

+++ binutils-2.19/configure	2008-11-23 16:22:02.000000000 +0100

@@ -5601,6 +5601,7 @@

   *-*-mingw* | *-*-cygwin ) RPATH_ENVVAR=PATH ;;

   *) RPATH_ENVVAR=LD_LIBRARY_PATH ;;

 esac

+RPATH_ENVVAR="cant_touch_this_nah_nah_nah"

 # On systems where the dynamic library environment variable is PATH,

 if test "$RPATH_ENVVAR" = PATH; then