arch: mark a set of target configurations as secondary

Those target configurations marked as "secondary" will be tested with a
lower probability in autobuilders. The intent is to focus the Buildroot
development (and bugfixes) on mainstream configurations, while still
keeping some other, less common, configurations.

This commit marks the following CPU architectures as secondary:

BR2_arcle (ARC little endian)
BR2_armeb (ARM big endian)
BR2_aarch64_be (AArch64 big endian)
BR2_hppa (PA-RISC)
BR2_loongarch64
BR2_m68k
BR2_microblazeel (Microblaze AXI, little endian)
BR2_microblazebe (Microblaze non-AXI, big endian)
BR2_mips (MIPS, big endian)
BR2_mipsel (MIPS, little endian)
BR2_mips64 (MIPS64 big endian)
BR2_or1k (OpenRISC)
BR2_powerpc (PowerPC, big endian)
BR2_powerpc64 (PowerPC64, big endian)
BR2_s390x
BR2_sh (SuperH)
BR2_sparc
BR2_sparc64
BR2_xtensa

In the following CPU architecture, only specific configurations are
marked secondary:

In BR2_armeb (ARM little endian),
- armv4 cores (arm920t, arm922t, fa526/626, strongarm sa110/sa1100)

In BR2_riscv:
- all 32-bit configurations
- 64-bit no-MMU

In BR2_i386 (x86 32-bit),
- i486, i586 and X1000 CPU

There was no strict rule established to build this list of secondary
configurations. This list was built mainly from the observation of
which architecture/CPUs are still widely used in the field
(disregarding its age), the quality of their upstream support and
the general relevance in the Buildroot project.

Signed-off-by: Julien Olivain <ju.o@free.fr>
[Arnout: use !BR2_HIDE_SECONDARY_TARGET_OPTIONS instead of
BR2_SHOW_SECONDARY_TARGET_OPTIONS]
Signed-off-by: Arnout Vandecappelle <arnout@rnout.be>

arch/Config.in.riscv

@@ -58,10 +58,12 @@ choice
 
 config BR2_RISCV_32
 	bool "32-bit"
+	depends on !BR2_HIDE_SECONDARY_TARGET_OPTIONS
 
 config BR2_RISCV_64
 	bool "64-bit"
 	select BR2_ARCH_IS_64
+	select BR2_RISCV_USE_MMU if BR2_HIDE_SECONDARY_TARGET_OPTIONS
 
 endchoice