build blemesh shell demo for linux

[gavin-yy]

I am trying to build the blemesh shell demo on linux platform. so it could be a ble mesh provisioner.
Maybe someone else also want to build this demo, so i pushed a commit here. commit

This commit want to keep nimble/host/mesh/src/shell.c not changed since it is in the nimble stack core directory.

Notes:

1. provisioner.c should work without marco BLE_MESH_CDB (for a simple demo)

Modification not be committed :

2. nimble/host/mesh/src/shell.c prints 'uint32_t' variable with "%lx" format, it cause compile warnning. (should switch to PRIx32 or %x)

3. bt_rand(void *buf, size_t len) calls ble_hs_hci_rand() to generate random numbers. My ble controller is buggly to support this.
   so i changed this function to read the random number from '/dev/urandom' for testing.

The test reuslt:

$sudo ./nimble-linux-blemesh-shell
...
mesh>provision 0 1 0
...
mesh>provision-adv dddd7cdfa1c215020000000000000000 0 2 0
...
send_prov_data: SessionKey: 608edf6774fdb2c01ae41c7dae59e21d
send_prov_data: Nonce: 20bf4ed33c6d6a8fe0a5f707c1
send_prov_data: DevKey: 9d588e3a77ec66919d770cb23cc7ae72
send_prov_data: net_idx 0, iv_index 0x00000000, addr 0x0002

prov_complete: key 9d588e3a77ec66919d770cb23cc7ae72, net_idx 0, num_elem 3, addr 0x0002>
...