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)

# GMP options

choice

    bool

    prompt "MPFR version"

# Don't remove next line

# CT_INSERT_VERSION_BELOW

config MPFR_V_2_4_2

    bool

    prompt "2.4.2"

config MPFR_V_2_4_1

    bool

    prompt "2.4.1"

config MPFR_V_2_4_0

    bool

    prompt "2.4.0"

config MPFR_V_2_3_2

    bool

    prompt "2.3.2 (OBSOLETE)"

    depends on OBSOLETE

config MPFR_V_2_3_1

    bool

    prompt "2.3.1 (OBSOLETE)"

    depends on OBSOLETE

endchoice

config MPFR_VERSION

    string

# Don't remove next line

# CT_INSERT_VERSION_STRING_BELOW

    default "2.4.2" if MPFR_V_2_4_2

    default "2.4.1" if MPFR_V_2_4_1

    default "2.4.0" if MPFR_V_2_4_0

    default "2.3.2" if MPFR_V_2_3_2

    default "2.3.1" if MPFR_V_2_3_1