fw_update: Introduce FirmwareInventory

Spec reference: [1]

Description:
FirmwareInventory is a class that manages all of the D-Bus
utilities for the firmware update functionality of each entry,
Fir

fw_update: Introduce FirmwareInventory

Spec reference: [1]

Description:
FirmwareInventory is a class that manages all of the D-Bus
utilities for the firmware update functionality of each entry,
FirmwareInventoryManager is used to manage multiple FirmwareInventory
instances.

Flow of the routine when one or more firmware device is added:
1. When one or more firmware device is added, the InventoryManager
instance will try to create a FirmwareInventory entry for the added
device, to accomplish this, it will need to fetch the necessary
information from various source (e.g. EM, device descriptors).

2. After the information is gathered, the InventoryManager will try to
obtain a Name for each firmware devices, to fit the different
platform condition, the name will be reference by the order below:
1. From Entity Manager (EM), if the endpoint has a specific EM
configuration with "Name" property listed.
2. From device descriptors, if the device descriptor contains a
vendor defined descriptor with "OpenBMC.Name" titled.
3. Spawn a default one, named "Firmware_Device_<Endpoint ID>".

3. The InventoryManager will then invoke
`FirmwareInventoryManager::createFirmwareInventory` to create a
firmware inventory entry.

Properties managed by firmware Inventory:
1. Version
Version object that represents the firmware version
2. Association
Association object that represents the associations for the
firmware

The object path pattern of the firmware inventory entry is:
`/xyz/openbmc_project/software/<BoardName>_<SoftwareName>_<SoftwareID>`

Where:
- `<BoardName>` represents the board the device is on.
- `<SoftwareName>` is the name of the firmware device obtained from the
InventoryManager.
- `<SoftwareID>` is a 4-byte random number to help consumer
distinguish from the new/old objects of a inventory item.
For example:
`server_board_slot_1_VR_2603`
The new Activation object and its related interfaces needs to
resides on a new objectPath, hence the softwareId is appended to
the path.

Test results:
- Build passed
- Successfully create firmware inventory entries
on Yosemite V4

[1]: https://github.com/openbmc/docs/blob/master/designs/code-update.md

Change-Id: Idebd7d013c82c60f08309a1860d5de1deeb3829a
Signed-off-by: Unive Tien <unive.tien.wiwynn@gmail.com>
Signed-off-by: Carter Chen <carter.chen.wiwynn@gmail.com>

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