M28: Cleanup memsize.o OOT build

M28: Cleanup memsize.o OOT build

The current way memsize.c is built just made a symlink into the directory
with SPL and then compiled it like any other file there. This was bad as that
broke the out-of-tree build.

The new way introduced in this patch uses the standard spl/Makefile methods
(CONFIG_SPL_LIBCOMMON_SUPPORT / CONFIG_SPL_LIBGENERIC_SUPPORT) to let files in
common/ be built. Because common/Makefile says memsize.c is always built (SPL
and non-SPL build), this fixes our issue with memsize.c out-of-tree build.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Detlev Zundel <dzu@denx.de>
Cc: Stefano Babic <sbabic@denx.de>