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)

--- a/extra/scripts/unifdef.c

+++ b/extra/scripts/unifdef.c

@@ -206,7 +206,7 @@

 static void             error(const char *);

 static int              findsym(const char *);

 static void             flushline(bool);

-static Linetype         getline(void);

+static Linetype         get_line(void);

 static Linetype         ifeval(const char **);

 static void             ignoreoff(void);

 static void             ignoreon(void);

@@ -512,7 +512,7 @@

 	for (;;) {

 		linenum++;

-		lineval = getline();

+		lineval = get_line();

 		trans_table[ifstate[depth]][lineval]();

 		debug("process %s -> %s depth %d",

 		    linetype_name[lineval],

@@ -526,7 +526,7 @@

  * help from skipcomment().

  */

 static Linetype

-getline(void)

+get_line(void)

 {

 	const char *cp;

 	int cursym;