[RDK-52502] added new mount/unmount APIs.

[goruklu]

Description

initial implementation of mount & unmount operations (only works on devices with kernel version > 5.2

Test Procedure

Use the following commands to mount and unmount directories in already running containers:

DobbyTool mount com.sky.as.apps_com.bskyb.epgui  /mnt/usb /tmp/newmount ro,bind
DobbyTool unmount com.sky.as.apps_com.bskyb.epgui /tmp/newmount

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Other (doesn't fit into the above categories - e.g. documentation updates)

Requires Bitbake Recipe changes?

• The base Bitbake recipe (meta-rdk-ext/recipes-containers/dobby/dobby.bb) must be modified to support the changes in this PR (beyond updating SRC_REV)

[bgray-sky]

Looks good to me.