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-4.0.0/libstdc++-v3/include/c_std/std_cstdio.h-orig	2005-04-29 00:08:41.000000000 -0500

 +++ gcc-4.0.0/libstdc++-v3/include/c_std/std_cstdio.h	2005-04-29 00:08:45.000000000 -0500

 @@ -142,7 +142,7 @@

    using ::vsprintf;

  }

 -#if _GLIBCXX_USE_C99

 +#if _GLIBCXX_USE_C99 || defined(__UCLIBC__)

  #undef snprintf

  #undef vfscanf