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>

 	Hack from Debian to hopefully get sandbox working on mips

 <`Kumba> ths: given the SHN_UNDEF thing is a hack, what's the preferred solution?

 <ths> For fakeroot the simplest trigger is "fakeroot file /bin/ls".

 <ths> `Kumba: I haven't found a better one yet.

 <ths> Probably marker symbols around the stub section, and then exclude it from the link map.

 <ths> This needs a ld change.

 diff -durN glibc-2.10.1.orig/elf/do-lookup.h glibc-2.10.1/elf/do-lookup.h

 --- glibc-2.10.1.orig/elf/do-lookup.h	2009-03-30 23:14:32.000000000 +0200

 +++ glibc-2.10.1/elf/do-lookup.h	2009-11-13 00:51:36.000000000 +0100

 @@ -248,6 +248,12 @@

  		}

  	      /* FALLTHROUGH */

  	    case STB_GLOBAL:

 +#ifdef __mips__

 +	      /* HACK: MIPS marks its lazy evaluation stubs with SHN_UNDEF

 +		 symbols, we skip them. */

 +	      if (sym->st_shndx == SHN_UNDEF)

 +		break;

 +#endif

  	      /* Global definition.  Just what we need.  */

  	      result->s = sym;

  	      result->m = (struct link_map *) map;