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>

 diff -dur strace-4.5.16.orig/process.c strace-4.5.16/process.c

 --- strace-4.5.16.orig/process.c	2007-01-11 23:08:38.000000000 +0100

 +++ strace-4.5.16/process.c	2007-07-14 19:19:58.000000000 +0200

 @@ -2685,7 +2685,6 @@

         { 4*REG_GBR,            "4*REG_GBR"                             },

         { 4*REG_MACH,           "4*REG_MACH"                            },

         { 4*REG_MACL,           "4*REG_MACL"                            },

 -       { 4*REG_SYSCALL,        "4*REG_SYSCALL"                         },

         { 4*REG_FPUL,           "4*REG_FPUL"                            },

         { 4*REG_FPREG0,         "4*REG_FPREG0"                          },

         { 4*(REG_FPREG0+1),     "4*REG_FPREG1"                          },