classes/populate_sdk_base: enable adding custom commands to SDK install script

Add an SDK_POST_INSTALL_COMMAND variable which allows additional
commands to be added towards the end of the SDK install script, for e.g.
additional processing that needs to be done as part of installing the
SDK.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>