conf/bitbake.conf: default HOMEPAGE to blank instead of unknown

The default value for HOMEPAGE of "unknown" has been in place since the
early OE-Classic days, but it doesn't really make sense - "unknown" is
not a valid URL and it just means we have to explicitly check for this
hardcoded string if we're displaying the value in some form of UI, such
as Toaster.

This has required some changes to the packaging classes as they
previously did not expect the value to be blank.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
python-smartpm: really ignore conflicts during install with --attempt

The original patch added in OE-Core commit
bdf07b1698d228dc7ff555199a269b1ff8ceca19 was supposed to ignore
conflicts, but it was unable to do so because it wasn't raising errors
in the right place. When the --attempt option is used (as is done in
complementary package installation for RPM), raise errors immediately
on conflicts, catch errors at the right point so that requested packages
and their dependencies can be ignored, and print appropriate warnings
when doing so.

Fixes [YOCTO #5313].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>