lib/oe/package_manager: Do not pass stderr to package manager as an argument

The cmd redirected stderr to stdout that was then assigned to variable
with pkgs to install. Then this variable was passed to package manager
that then tried to install it and generated confusing warnings.

For example this variable could contain:
| ['(en_US.UTF-8)', 'LC_ALL:', 'bash:', 'cannot', 'change', 'locale', 'setlocale:', 'warning:']

and the warning was:

| WARNING: addon-bci-1.0-r0 do_populate_sdk: Unable to install packages.
| Command 'PATH/usr/bin/opkg ... install (en_US.UTF-8) LC_ALL: bash:
| cannot change locale setlocale: warning:' returned 255:
| Collected errors:
|  * opkg_prepare_url_for_install: Couldn't find anything to satisfy '(en_US.UTF-8)'.

In this change I remove stderr redirection to stdout and pass it to
bb.note instead.

Signed-off-by: Tomasz Dziendzielski <tomasz.dziendzielski@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>