populate_sdk_ext.bbclass: Fix the derivate sdk build error from an published eSDK

populate_sdk_ext.bbclass: Fix the derivate sdk build error from an published eSDK

When you install the eSDK in publish mode and then try to build a
derivative sdk using devtool build-sdk, the following error happens

| NOTE: Generating sstate task list...
| NOTE: Generating sstate-cache...
| NOTE: Generating sstate-cache...
| DEBUG: Python function copy_buildsystem finished
| NOTE: Executing install_tools ...
| DEBUG: Executing shell function install_tools
| install: cannot stat /core/meta/files/ext-sdk-prepare.py':
No such file or directory

This patch will fix the error by installing ext-sdk-prepare.py in
publish mode, so that derivative sdk can be generated.

Signed-off-by: Manjukumar Matha <manjukumar.harthikote-matha@xilinx.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>