Stable: Small improvements

Changes:

• Added new API where we can enumerate devices with target id as a key
• Added new CLI command --json-by-target-id (prints JSON dict with TargetIDs as keys)

Example:

{
    "02400203D94B0E7724B7F3CF": {
        "mount_point": "I:",
        "platform_name": "K64F",
        "platform_name_unique": "K64F[0]",
        "serial_port": "COM61",
        "target_id": "02400203D94B0E7724B7F3CF",
        "target_id_mbed_htm": "02400203D94B0E7724B7F3CF",
        "target_id_usb_id": "02400203D94B0E7724B7F3CF"
    },
    "101000000000000000000002F7F0D9F98dbdc24b9e28ac87cfc4f23c4c57438d": {
        "mount_point": "G:",
        "platform_name": "LPC1768",
        "platform_name_unique": "LPC1768[0]",
        "serial_port": "COM5",
        "target_id": "101000000000000000000002F7F0D9F98dbdc24b9e28ac87cfc4f23c4c57438d",
        "target_id_mbed_htm": "101000000000000000000002F7F0D9F98dbdc24b9e28ac87cfc4f23c4c57438d",
        "target_id_usb_id": "A000000001"
    }
}

• Added Doxygen documentation for mbed-ls code.