arch/Config.in: BR2_BINFMT_FLAT remove architecture dependencies

The commit [1] removed the Config.in.host file and moved the arm, sh,
sparc, xtensa and riscv-64 architectures dependencies.  But this list
doesn't contain all elf2flt supported architectures and this breaks
the host-gcc-initial build for m68k.

Since BR2_BINFMT_FLAT is only available on no MMU case, we can rely on
BR2_USE_MMU option and remove the architecture list.

Note: SH without MMU support has been removed by [2], sparc requires
an MMU since at least [3].

Fixes:

  http://autobuild.buildroot.net/results/15d62edfc4b54c0b40b9d3961b797bce4e307919/

[1] 9db5eb258cf492567bac33a33cb606f14045639d
[2] 04ea3d38dd76f4e96f4961143d674aff59b13d3f
[3] 29563047e074a7c0b73c9e6b629d08bac1e07cb5

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

arch/Config.in

@@ -388,7 +388,6 @@ config BR2_BINFMT_ELF
 
 config BR2_BINFMT_FLAT
 	bool "FLAT"
-	depends on BR2_arm || BR2_sh || BR2_sparc || BR2_xtensa || BR2_RISCV_64
 	depends on !BR2_USE_MMU
 	help
 	  FLAT binary is a relatively simple and lightweight executable