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 -ur uClibc-0.9.29/libc/inet/resolv.c uClibc-0.9.29-patched/libc/inet/resolv.c

 --- uClibc-0.9.29/libc/inet/resolv.c	2007-04-23 12:01:05.000000000 -0500

 +++ uClibc-0.9.29-patched/libc/inet/resolv.c	2007-05-09 18:05:33.563404419 -0500

 @@ -1700,7 +1700,7 @@

  int gethostent_r(struct hostent *result_buf, char *buf, size_t buflen,

  	struct hostent **result, int *h_errnop)

  {

 -    int ret;

 +    int ret = HOST_NOT_FOUND;

      __UCLIBC_MUTEX_LOCK(mylock);

      if (__gethostent_fp == NULL) {