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>