net: lwm2m: remove predefined firmware buffer from firmware_pull

Now that we can access resource data in the lwm2m subsys, let's use
the user provided firmware push buffer (5/0/0) to also store the
firmware pull data.

This way the size of the firmware pull buffer is completely up to the
application.

NOTE: This patch adds a 64 byte firmware buffer to the lwm2m_client
sample for this purpose.

Signed-off-by: Michael Scott <michael.scott@linaro.org>