m32r: fix endianness constraints

The m32r Kconfig provides both CPU_BIG_ENDIAN and CPU_LITTLE_ENDIAN
configuration options.  As they are user-selectable and independent,
this allows invalid configurations:

  - All m32r defconfigs build a big endian kernel, but CPU_BIG_ENDIAN is
    not set, causing compiler warnings like:

	include/linux/byteorder/big_endian.h:7:2: warning: #warning inconsistent configuration, needs CONFIG_CPU_BIG_ENDIAN [-Wcpp]
	 #warning inconsistent configuration, needs CONFIG_CPU_BIG_ENDIAN
	  ^

  - Since commit 5bdfca64 ("m32r: define CPU_BIG_ENDIAN"),
    building an allmodconfig or allyesconfig enables both
    CONFIG_CPU_BIG_ENDIAN and CONFIG_CPU_LITTLE_ENDIAN.
    While this did get rid of the warning above, both options are
    obviously mutually exclusive.

Fix this by making only CPU_LITTLE_ENDIAN configurable by the user, as
before, and by making sure exactly one of CPU_BIG_ENDIAN and
CPU_LITTLE_ENDIAN is always enabled.

Link: http://lkml.kernel.org/r/1509361505-18150-1-git-send-email-geert@linux-m68k.org


Fixes: 5bdfca64 ("m32r: define CPU_BIG_ENDIAN")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>