pppd: needs mmu and shared objects

The plugin infrastructure is based on shared objects so it won't build
for static-only scenarios.
And the daemon uses fork() so MMU is required.

[Peter: move below suboptions]
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>