common/board_f: move board_init_f_mem() from #else CONFIG_X86

common/board_f: move board_init_f_mem() from #else CONFIG_X86

Purpose of this change is to make it possible to re-use code currently
used on X86 solely for other architectures. For example:
 * init_sequence_f_r
 * board_init_f_r

Even though board_init_f_mem() has nothing to do with any particular
architecture it won't work (at least in current implementation) for X86.

This is because on X86 "gd" is an alias to function get_fs_gd_ptr(),
thus we cannot assign anything to it.

So this change separates selection of board_init_f_mem() from X86 while
keeping it disabled for X86 still.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>