package_rpm.bbclass: Fix attemptonly install failures

package_rpm.bbclass: Fix attemptonly install failures

During an attemptonly install, if you try to install two packages that
conflict with each other, a message is generated:

 error: Failed dependencies:
     dropbear conflicts with openssh-6.0p1-r4.ppc603e
     dropbear conflicts with openssh-sshd-6.0p1-r4.ppc603e

This is placed onto stderr (vs stdout), which was not being redirected.
This allowed the log-check code to find the magic key of "Failed" causing
the package installation process to fail.  Instead the proper behavior is to
simply ignore the error and proceed with the install.

(From OE-Core rev: 16d8d7fdda4ae7d108f5d50cde3f6104e231bd23)

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>