build: make sure modules are modposted after initramfs image is built

Otherwise the modpost steps for individual modules that are compiled
manually (using make package/<name_of_module>/install) will give warning
of missing symbols when that module depends other modules.

This is caused by the Module.symvers file not containing any symbols
anymore of external modules when the initramfs image is built without
specifically giving the modules target.

Signed-off-by: Tjalling Hattink <t.hattink@fugro.nl>

SVN-Revision: 42773