samples: net: lwm2m: bootstrap support changes:

When LWM2M_RD_CLIENT_SUPPORT_BOOTSTRAP is enabled:
- Mark the first server as a bootstrap server
- Create second server and security instances (used by bootstrap
  server)

This allows bootstrap support to be tested out of the box with
the following:
- setup Leshan bss-server-demo (bootstrap server) on port 5783
- setup Leshan server-demo (lwm2m server) on port 5683
- add an entry into the bootstrap server:
  LWM2M Server=coap://[2001:db8::2]:5683


- build the sample like so:
  west build -t run -b qemu_x86 \
    -s zephyr/samples/net/lwm2m_client/ -- \
    -DCONFIG_LWM2M_RD_CLIENT_SUPPORT_BOOTSTRAP=y \
    -DCONFIG_LWM2M_PEER_PORT=5783

Signed-off-by: Michael Scott <mike@foundries.io>