populate_sdk_base: remove unneeded dirs such as /dev

We met a problem that core-image-tiny-initramfs's SDK cannot be
installed. The error message is like below.

  tar: ./sysroots/core2-64-poky-linux/dev/console: Cannot mknod: Operation not permitted

In fact, the '/dev' direcotry is not needed by SDK. So remove it.

This patches uses a variable, SDK_PRUNE_SYSROOT_DIRS, to hold useless dir entries
so that it could be extended. For example, '/usr/bin' could be added if wanted.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>