image/populate_sdk: Support usrmerge in toolchain SDK builds

image/populate_sdk: Support usrmerge in toolchain SDK builds

SDKs which aren't built from an image recipe should also come with
drop-in symlinks for `/bin`, `/lib` and `/sbin`. Support this by moving
the common functionality into `populate_sdk_base` and enabling the
symlink creation when the `usrmerge` DISTRO_FEATURE is on.

Signed-off-by: Philip Lorenz <philip.lorenz@bmw.de>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>