Allow configuration of platform-specific interfaces

[pyadvichuk]

- Why it is required
since SONiC master switches ifupdown package to the new implementation (ifupdown2), it is required to change the configuration of a platform-specific interface for wedge100bf_32x and wedge100bf_65x platforms (bc of ifupdown2 doesn't support auto mode for inet6 protocol).

Also, need to make some refactoring and remove if platform == smth then.. from the system level scripts.

- What I did

• removed customization of /usr/bin/interfaces-config.sh
• explicitly created directory /etc/network/interfaces.d
• added "source" to the /etc/network/interfaces generation template (to include platform-specific interfaces processing)
• added platform-specific interfaces config itself (for wedge100bf_32x and wedge100bf_65x)
• fixed testcase in sonic-config-engine

- How to verify it

• build image for wedge100bf_32x
• perform sudo config reload -y on new installation
• check the correct configuration of usb0 interface

- Description for the changelog

• Allow configuration of platform-specific interfaces

@akokhan, @mkbalani: please review this first

@lguohan: please review and merge this

[msftclas]

All CLA requirements met.

[mkbalani]

@pyadvichuk without auto config in " iface usb0 inet6 auto" is the BMC interface configured correctly. What is the default when auto/dhcp/static is not used. We need the BMC to come with some IPv6 address config

[lguohan]

@pyadvichuk , please sign cla.

[pyadvichuk]

@pyadvichuk without auto config in " iface usb0 inet6 auto" is the BMC interface configured correctly. What is the default when auto/dhcp/static is not used. We need the BMC to come with some IPv6 address config

@mkbalani, by default static link level v6 address is assigned. I've checked REST over this interface by sensors command. Everything looks good.

Changes not needed

[akokhan]

@lguohan is there anything that blocks us from merging this? Thanks.