package_manager/ipk: do not pipe stderr to stdout

Some opkg commands print an error during cleanup when the tmp_dir
does not exist and an attempt is made to delete it. The error messages
are harmless and the opkg commands eventually succeed.
When these commands are run and stderr is piped to stdout, the error
messages may clobber the stdout and cause unexpected results while
parsing the output of the command. Therefore, when parsing the output
of a command, do not pipe stderr to stdout. Instead, capture stderr
and stdout separately, and upon success, send stderr to bb.note().

Signed-off-by: Shruthi Ravichandran <shruthi.ravichandran@ni.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>