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)

# Super-H specific configuration file

config ARCH_sh

    select ARCH_SUPPORTS_32

    select ARCH_DEFAULT_32

    select ARCH_USE_MMU

    select ARCH_SUPPORTS_BOTH_ENDIAN

    select ARCH_DEFAULT_LE

    help

      The Super-H architecture, as defined by:

        http://www.renesas.com/fmwk.jsp?cnt=superh_family_landing.jsp&fp=/products/mpumcu/superh_family/

choice

    bool

    prompt "Variant"

config ARCH_SH_SH3

    bool

    prompt "sh3"

config ARCH_SH_SH4

    bool

    prompt "sh4"

config ARCH_SH_SH4A

    bool

    prompt "sh4a"

endchoice

config ARCH_SH_VARIANT

    string

    default "sh3"   if ARCH_SH_SH3

    default "sh4"   if ARCH_SH_SH4

    default "sh4a"  if ARCH_SH_SH4A