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>

 --- trunk/src/acos.c	2009/12/08 16:49:43	733

 +++ trunk/src/acos.c	2009/12/09 20:51:38	734

 @@ -188,10 +188,7 @@

    else

      rnd_im = rnd_im == GMP_RNDU ? GMP_RNDD

        : rnd_im == GMP_RNDD ? GMP_RNDU

 -#if MPFR_VERSION_MAJOR >= 3

 -      : rnd_im == GMP_RNDA ? GMP_RNDZ

 -#endif

 -      : rnd_im;

 +      : rnd_im; /* both RNDZ and RNDA map to themselves for -asin(z) */

    rnd1 = RNDC(GMP_RNDN, rnd_im);

    mpfr_init2 (pi_over_2, p);

    for (;;)