Disable "Conflicting CPU architectures" link warnings

I added the --no-warn-mismatch linker flag to silence these warnings
which are generated when linking in the bare SoftDevice binary to
NRF51822 binaries.
Some cleanup to -whole-archive fix

I defined an all_objs_from_mbed() macro in gcc4mbed.mk and then call it
from the command list of the link rule in device-common.mk  This makes
it a bit clearer what is happening when you read the link rule.