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)

 # MPC options

 choice

     bool

     prompt "MPC version"

 # Don't remove next line

 # CT_INSERT_VERSION_BELOW

 config MPC_V_0_8_1

     bool

     prompt "0.8.1"

 config MPC_V_0_7

     bool

     prompt "0.7"

 config MPC_V_0_6

     bool

     prompt "0.6 (OBSOLETE)"

     depends on OBSOLETE

 endchoice

 config MPC_VERSION

     string

 # Don't remove next line

 # CT_INSERT_VERSION_STRING_BELOW

     default "0.8.1" if MPC_V_0_8_1

     default "0.7" if MPC_V_0_7

     default "0.6" if MPC_V_0_6