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 uClibc-0.9.28.1.orig/libc/sysdeps/linux/common/Makefile uClibc-0.9.28.1/libc/sysdeps/linux/common/Makefile

 --- uClibc-0.9.28.1.orig/libc/sysdeps/linux/common/Makefile	2007-01-26 00:54:19.000000000 +0100

 +++ uClibc-0.9.28.1/libc/sysdeps/linux/common/Makefile	2007-02-01 08:44:37.000000000 +0100

 @@ -25,6 +25,10 @@

  SRCS := $(filter-out sbrk.c,$(SRCS))

  endif

 +ifneq ($(strip $(UCLIBC_HAS_LFS)),y)

 +SRCS := $(filter-out readahead.c,$(SRCS))

 +endif

 +

  ifneq ($(strip $(UCLIBC_HAS_SSP)),y)

  SRCS := $(filter-out ssp.c,$(SRCS))

  endif