logs: Add support for reading number of entries since an index for a particular log

[vrahane]

Related mcumgr PR: apache/mynewt-mcumgr#172
Related mynewt-core PR: apache/mynewt-core#3168

Example:
Successful/Valid case:

$newtmgr --connstring peer_name="my_dev" --conntype oic_ble log num_entries reboot_log 0
Number of entries: 43

Error case:

<Invalid log>
$newtmgr --connstring peer_name="my_dev" --conntype oic_ble log num_entries reboot 0
error: 5

<Invalid index>
$newtmgr --connstring peer_name="my_dev" --conntype oic_ble log num_entries reboot 4000
error: 5