ixp4xx-npe: fix COMPATIBLE_MACHINE

ERROR: Nothing PROVIDES 'ixp4xx-npe-native' (but /build/jenkins/angstrom-v2014.06/machine/nslu2be/sources/meta-nslu2/recipes-kernel/ixp4xx/ixp4xx-npe_2.4.bb DEPENDS on or otherwise requires it)
ERROR: ixp4xx-npe-native was skipped: incompatible with machine nslu2be (not in COMPATIBLE_MACHINE)
NOTE: Runtime target 'ixp4xx-npe' is unbuildable, removing...
Missing or unbuildable dependency chain was: ['ixp4xx-npe','ixp4xx-npe-native']
NOTE: Runtime target 'angstrom-packagegroup-boot' is unbuildable, removing...
Missing or unbuildable dependency chain was: ['angstrom-packagegroup-boot', 'packagegroup-boot', 'ixp4xx-npe', 'ixp4xx-npe-native']
ERROR: Required build target 'systemd-image' has no buildable providers.
Missing or unbuildable dependency chain was: ['systemd-image', 'angstrom-packagegroup-boot', 'packagegroup-boot', 'ixp4xx-npe','ixp4xx-npe-native']

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>