I personally dislike "common" variables, as they hid bugs a few times
for me. Thus, ppsi has "-fno-common" by now.  This commit adds
extern to memlayout.h and creates the variables in dev/sdb.c .
This builds whether or not -fno-common is there and has no effect
on a ptp-noposix build.

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>