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)

 # Compute x86-specific values

 # This one really needs a little love! :-(

 CT_DoArchTupleValues() {

     # Override the architecture part of the tuple:

     if [ "${CT_ARCH_64}" = "y" ]; then

         CT_TARGET_ARCH=x86_64

     else

         arch="${CT_ARCH_ARCH}"

         [ -z "${arch}" ] && arch="${CT_ARCH_TUNE}"

         case "${arch}" in

             "")                           CT_TARGET_ARCH=i386;;

             i386|i486|i586|i686)          CT_TARGET_ARCH="${arch}";;

             winchip*)                     CT_TARGET_ARCH=i486;;

             pentium|pentium-mmx|c3*)      CT_TARGET_ARCH=i586;;

             pentiumpro|pentium*|athlon*)  CT_TARGET_ARCH=i686;;

             *)                            CT_TARGET_ARCH=i586;;

         esac

     fi

 }