[Nokia ixs7215] Add SW assist for platform entropy & fix inband mgmt

[dflynn-Nokia]

This PR provides the following updates to the Nokia ixs7215 platform

1. The Marvell Armada-38x SOC requires SW assistance to improve the system
   entropy value available early on in the Sonic boot sequence.
2. The Nokia ixs7215 platform does not have a dedicated Out-Of-Band (OOB) mgmt
   port and thus requires additional logic to optionally support configuring
   front panel port 48 as an In-Band mgmt port. This commit provides additional
   logic to manage and maintain the operation of this In-Band mgmt port.

These changes are similar to those made for the Marvell et6448m platform as provided in PR# #5749

- Why I did it

• Improve random number generation during early Sonic initialization by providing SW updates to Linux entropy value.
• Improve handling of platform In-Band management port

- How I did it

• Provide logic to update the entropy value via updates to /dev/random
• Provide logic to monitor the state of the In-Band mgmt port and restart when required.

- How to verify it

• Confirm presence of /etc/entropy.py
• Confirm via ps command that inband_mgmt.sh is running on the Sonic host

- Which release branch to backport (provide reason below if selected)

• 201811
• 201911
• 202006
• 202012

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)