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>

 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/