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>

 diff --git a/mpz/remove.c b/mpz/remove.c

 --- a/mpz/remove.c

 +++ b/mpz/remove.c

 @@ -23,7 +23,7 @@

  unsigned long int

  mpz_remove (mpz_ptr dest, mpz_srcptr src, mpz_srcptr f)

  {

 -  mpz_t fpow[40];		/* inexhaustible...until year 2020 or so */

 +  mpz_t fpow[GMP_LIMB_BITS];		/* Really MP_SIZE_T_BITS */

    mpz_t x, rem;

    unsigned long int pwr;

    int p;

 @@ -69,7 +69,7 @@

        mpz_set (dest, x);

      }

 -  pwr = (1 << p) - 1;

 +  pwr = (1L << p) - 1;

    mpz_clear (fpow[p]);

 diff --git a/mpz/remove.c b/mpz/remove.c

 --- a/mpz/remove.c

 +++ b/mpz/remove.c

 @@ -80,7 +80,7 @@

        mpz_tdiv_qr (x, rem, dest, fpow[p]);

        if (SIZ (rem) == 0)

  	{

 -	  pwr += 1 << p;

 +	  pwr += 1L << p;

  	  mpz_set (dest, x);

  	}

        mpz_clear (fpow[p]);