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>

 # MIPS specific config options

 choice

     bool

     prompt "ABI"

 config ARCH_mips_o32

     bool

     prompt "o32"

     depends on ARCH_32

     help

       This is the -mabi=32 gcc option.

 config ARCH_mips_n32

     bool

     prompt "n32"

     depends on ARCH_64

     help

       This is the -mabi=n32 gcc option.

 config ARCH_mips_n64

     bool

     prompt "n64"

     depends on ARCH_64

     help

       This is the -mabi=64 gcc option.

 # Not supported on Linux:

 # o64   : seems related to *BSD

 # eabi  : seems related to bare-metal

 endchoice

 config ARCH_mips_ABI

     string

     default "32"    if ARCH_mips_o32

     default "n32"   if ARCH_mips_n32

     default "64"    if ARCH_mips_n64