netbase: fix PACKAGE_ARCH

OE renames MACHINE=foo-bar to foo_bar to work around target tupple and other problems, so use MACHINE_ARCH for machine specific packages.

Failure mode:

ls /build/v2012.12/build/tmp-angstrom_v2012_12-eglibc/deploy/ipk/ | grep a13
olinuxino_a13
olinuxino-a13

ls /build/v2012.12/build/tmp-angstrom_v2012_12-eglibc/deploy/ipk/olinuxino-a13/
netbase_5.0-r1.0_olinuxino-a13.ipk  netbase-dbg_5.0-r1.0_olinuxino-a13.ipk  netbase-dev_5.0-r1.0_olinuxino-a13.ipk  netbase-doc_5.0-r1.0_olinuxino-a13.ipk

The rest of the machine specific packages are in the (correct) olinuxino_a13 directory

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>