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)

 diff -dur ELFkickers.orig/sstrip/Makefile ELFkickers/sstrip/Makefile

 --- ELFkickers.orig/sstrip/Makefile	2001-03-24 12:58:27.000000000 +0100

 +++ ELFkickers/sstrip/Makefile	2007-05-31 21:17:18.000000000 +0200

 @@ -1,7 +1,7 @@

  #  Makefile for sstrip

  sstrip: sstrip.c

 -	gcc -ggdb -Wall -W -o sstrip sstrip.c

 +	$(CC) -Wall -W -o sstrip sstrip.c

  clean:

  	rm -f sstrip