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 Gentoo.

 -= BEGIN original header =-

 -= END original header =-

 diff -durN uClibc-0.9.30.orig/libc/inet/resolv.c uClibc-0.9.30/libc/inet/resolv.c

 --- uClibc-0.9.30.orig/libc/inet/resolv.c	2008-11-02 01:25:33.000000000 +0100

 +++ uClibc-0.9.30/libc/inet/resolv.c	2009-02-07 09:57:59.000000000 +0100

 @@ -1788,7 +1788,7 @@

  int gethostent_r(struct hostent *result_buf, char *buf, size_t buflen,

  	struct hostent **result, int *h_errnop)

  {

 -	int ret;

 +	int ret = HOST_NOT_FOUND;

  	__UCLIBC_MUTEX_LOCK(mylock);

  	if (__gethostent_fp == NULL) {