kernel/linux: fix using custom location

Currently, extract and patch are skipped as thus:
- using a custom directory of pre-installed headers
- a correctly named directory already exists

Otherwise, extract and patch are done.

The current second condition is wrong, because it allows the following
sequence to happen:
- a non-custom kernel is used
- a previous build only partially extracted the non-custom sources
- that p[revious build broke during extraction (eg. incomplete tarball...)
- a subsequent build will find a properly named directory, and will
thus skip extract and patch, which is wrong

Fix that by following the conditions in this table:

Type | Extract | Patch
----------------------+---------+-------
Pre-installed headers | N | N
custom directory | N | N
custom tarball | Y | N
mainstream tarball | Y | Y

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: David Holsgrove <david.holsgrove@xilinx.com>

 Original patch from: mpf_set_str.c.4.diff

 -= BEGIN original header =-

 -= END original header =-

 diff -durN gmp-4.2.4.orig/mpf/set_str.c gmp-4.2.4/mpf/set_str.c

 --- gmp-4.2.4.orig/mpf/set_str.c	2008-08-25 16:11:37.000000000 +0200

 +++ gmp-4.2.4/mpf/set_str.c	2009-03-08 18:36:16.000000000 +0100

 @@ -137,7 +137,12 @@

        c = (unsigned char) *++str;

      }

 +  /* Default base to decimal.  */

 +  if (base == 0)

 +    base = 10;

 +

    exp_base = base;

 +

    if (base < 0)

      {

        exp_base = 10;

 @@ -165,10 +170,6 @@

  	return -1;

      }

 -  /* Default base to decimal.  */

 -  if (base == 0)

 -    base = 10;

 -

    /* Locate exponent part of the input.  Look from the right of the string,

       since the exponent is usually a lot shorter than the mantissa.  */

    expptr = NULL;