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 -durN gcc-3.4.6.orig/gcc/config/sh/sh.h gcc-3.4.6/gcc/config/sh/sh.h

 --- gcc-3.4.6.orig/gcc/config/sh/sh.h	2006-01-12 02:20:32.000000000 +0100

 +++ gcc-3.4.6/gcc/config/sh/sh.h	2007-08-15 23:03:09.000000000 +0200

 @@ -422,6 +422,10 @@

  do {									\

    if (LEVEL)								\

      flag_omit_frame_pointer = -1;					\

 +  if (LEVEL <= 2)							\

 +    {									\

 +      flag_reorder_blocks = 0;						\

 +    }									\

    if (SIZE)								\

      target_flags |= SPACE_BIT;						\

    if (TARGET_SHMEDIA && LEVEL > 1)					\