populate_sdk_base: inherit nopackages

populate_sdk_base: inherit nopackages

Since this bbclass sets PACKAGES = "", inherit the nopackages
class to skip the various packaging functions which wouldn't
do anything anyway.

This fixes errors from buildhistory changes where packages-split would be empty.

e.g. meta-toolchain build now fails with:
| DEBUG: Executing shell function buildhistory_list_pkg_files
| find: ".../meta-toolchain/1.0/packages-split/*": No such file or directory
| WARNING: exit code 1 from a shell command.
| DEBUG: Python function buildhistory_emit_pkghistory finished

Signed-off-by: Martin Jansa <martin.jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>