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/Rules.mak uClibc-0.9.30/Rules.mak

--- uClibc-0.9.30.orig/Rules.mak	2008-11-12 13:24:16.000000000 +0100

+++ uClibc-0.9.30/Rules.mak	2009-02-07 09:57:59.000000000 +0100

@@ -502,6 +502,7 @@

 endif

 CFLAGS += $(call check_gcc,-std=gnu99,)

+CFLAGS += $(call check_gcc,-fgnu89-inline,)

 LDFLAGS_NOSTRIP:=$(CPU_LDFLAGS-y) -Wl,-shared \

 	-Wl,--warn-common -Wl,--warn-once -Wl,-z,combreloc