airoha: fix imagebuilder generation by introducing generic target

airoha: fix imagebuilder generation by introducing generic target

The generic imagebuilder does not have a generic in the name, although
this is the default naming scheme. Apply the same fix as for the octeon
target. Thanks to @dangowrt for reporting that the same issues applies
also for the airoha target.

Before the fix:
openwrt-imagebuilder-airoha.Linux-x86_64.tar.xz

After:
openwrt-imagebuilder-airoha-generic.Linux-x86_64.tar.xz

Signed-off-by: Nick Hainke <vincent@systemli.org>