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>

 --- glibc-2.3.6/linuxthreads/sysdeps/unix/sysv/linux/alpha/sysdep-cancel.h.orig	2008-06-19 15:31:45.000000000 +0300

 +++ glibc-2.3.6/linuxthreads/sysdeps/unix/sysv/linux/alpha/sysdep-cancel.h	2008-06-19 15:36:10.000000000 +0300

 @@ -54,6 +54,7 @@

  	bne	a3, SYSCALL_ERROR_LABEL;			\

  __LABEL($pseudo_ret)						\

  	.subsection 2;						\

 +	cfi_startproc;						\

  __LABEL($pseudo_cancel)						\

  	subq	sp, 64, sp;					\

  	cfi_def_cfa_offset(64);					\

 @@ -84,12 +85,13 @@

  	cfi_def_cfa_offset(0);					\

  __LABEL($syscall_error)						\

  	SYSCALL_ERROR_HANDLER;					\

 +	cfi_endproc;						\

  	.previous

  # undef PSEUDO_END

  # define PSEUDO_END(sym)					\

 -	.subsection 2;						\

  	cfi_endproc;						\

 +	.subsection 2;						\

  	.size sym, .-sym

  # define SAVE_ARGS_0	/* Nothing.  */