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/