generic-board: select SYS_GENERIC_BOARD for some architectures

We have done with the generic board conversion for all the boards
of ARC, Blackfin, M68000, MicroBlaze, MIPS, NIOS2, Sandbox, X86.

Let's select SYS_GENERIC_BOARD for those architectures, so we can
tell which architecture has finished the conversion at a glance.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alexey Brodkin <abrodkin@synopsys.com>
diff --git a/arch/sandbox/config.mk b/arch/sandbox/config.mk
index e477a84..b05a90f 100644
--- a/arch/sandbox/config.mk
+++ b/arch/sandbox/config.mk
@@ -2,7 +2,7 @@
 # SPDX-License-Identifier:	GPL-2.0+
 
 PLATFORM_CPPFLAGS += -D__SANDBOX__ -U_FORTIFY_SOURCE
-PLATFORM_CPPFLAGS += -DCONFIG_ARCH_MAP_SYSMEM -DCONFIG_SYS_GENERIC_BOARD
+PLATFORM_CPPFLAGS += -DCONFIG_ARCH_MAP_SYSMEM
 PLATFORM_LIBS += -lrt
 
 # Define this to avoid linking with SDL, which requires SDL libraries