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)

 # Menu for the native gdbserver

 config GDB_GDBSERVER

     bool

     prompt "gdbserver"

     default n

     depends on ! BARE_METAL

     help

       Build and install a gdbserver for the target, to run on the target.

 if GDB_GDBSERVER

 config GDB_GDBSERVER_STATIC

     bool

     prompt "Build a static gdbserver"

     default y

     help

       In case you have trouble with dynamic loading of shared libraries,

       you will find that a static gdbserver comes in handy.

 endif # GDB_GDBSERVER

 if BARE_METAL

 comment "In bare-metal, you'll need to   "

 comment "provide your own gdbserver stub."

 endif # BARE_METAL