The generic imagebuilder does not have a generic in the name, although
this is the default naming scheme. Use bcm53xx as template for this fix.
Before the fix:
openwrt-imagebuilder-octeon.Linux-x86_64.tar.xz
After:
openwrt-imagebuilder-octeon-generic.Linux-x86_64.tar.xz
Signed-off-by: Nick Hainke <vincent@systemli.org>