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>

 Respect environment CPPFLAGS when we run ./configure so we can inject

 random -D things without having to set CFLAGS/ASFLAGS

 diff -durN glibc-2.13.orig/Makeconfig glibc-2.13/Makeconfig

 --- glibc-2.13.orig/Makeconfig	2009-05-09 23:57:56.000000000 +0200

 +++ glibc-2.13/Makeconfig	2009-11-13 00:49:41.000000000 +0100

 @@ -697,6 +697,7 @@

  	   $(foreach lib,$(libof-$(basename $(@F))) \

  			 $(libof-$(<F)) $(libof-$(@F)),$(CPPFLAGS-$(lib))) \

  	   $(CPPFLAGS-$(<F)) $(CPPFLAGS-$(@F)) $(CPPFLAGS-$(basename $(@F)))

 +CPPFLAGS += $(CPPFLAGS-config)

  override CFLAGS	= -std=gnu99 $(gnu89-inline-CFLAGS) \

  		  $(filter-out %frame-pointer,$(+cflags)) $(+gccwarn-c) \

  		  $(sysdep-CFLAGS) $(CFLAGS-$(suffix $@)) $(CFLAGS-$(<F)) \