arch/Config.in: armeb no-MMU is not supported

elf2flt does not support ARM big-endian, so supporting Cortex M3/M4/M7
with armeb is not possibly.

Therefore this commit makes:

 - MMU mandatory on armeb
 - Prevents from seeing Cortex M3/M4/M7 on armeb

Fixes:

  http://autobuild.buildroot.net/results/9bca0cbfb6a66c455e74ad194526bca942665978/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit c9b39a81b566f827686d41682ae29ccfdaead4ef)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

arch/Config.in

@@ -49,7 +49,7 @@ config BR2_arm
 
 config BR2_armeb
 	bool "ARM (big endian)"
-	# MMU support is set by the subarchitecture file, arch/Config.in.arm
+	select BR2_ARCH_HAS_MMU_MANDATORY
 	help
 	  ARM is a 32-bit reduced instruction set computer (RISC)
 	  instruction set architecture (ISA) developed by ARM Holdings.

arch/Config.in.arm

@@ -259,6 +259,8 @@ config BR2_pj4
 	select BR2_ARM_CPU_ARMV7A
 	select BR2_ARCH_HAS_MMU_OPTIONAL
 
+# Cortex-M cores are only supported for little endian configurations
+if BR2_arm
 comment "armv7m cores"
 config BR2_cortex_m3
 	bool "cortex-M3"
@@ -275,6 +277,7 @@ config BR2_cortex_m7
 	select BR2_ARM_CPU_MAYBE_HAS_FPV5
 	select BR2_ARM_CPU_ARMV7M
 	select BR2_ARCH_NEEDS_GCC_AT_LEAST_5
+endif # BR2_arm
 endif # !BR2_ARCH_IS_64
 
 comment "armv8 cores"