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)

 --- gcc-4.1.2.orig/gcc/configure.ac	2006-11-13 23:09:55.000000000 +0100

 +++ gcc-4.1.2/gcc/configure.ac	2007-03-04 23:34:53.000000000 +0100

 @@ -2435,7 +2435,7 @@

  	tls_first_minor=14

  	tls_as_opt="-m64 -Aesame --fatal-warnings"

  	;;

 -  sh-*-* | sh[34]-*-*)

 +  sh-*-* | sh[34]*-*-*)

      conftest_s='

  	.section ".tdata","awT",@progbits

  foo:	.long	25

 --- gcc-4.1.2.orig/gcc/configure	2006-11-13 23:09:55.000000000 +0100

 +++ gcc-4.1.2/gcc/configure	2007-03-04 23:34:53.000000000 +0100

 @@ -14762,7 +14762,7 @@

  	tls_first_minor=14

  	tls_as_opt="-m64 -Aesame --fatal-warnings"

  	;;

 -  sh-*-* | sh[34]-*-*)

 +  sh-*-* | sh[34]*-*-*)

      conftest_s='

  	.section ".tdata","awT",@progbits

  foo:	.long	25