ar71xx: fix legacy image building

ar71xx: fix legacy image building

After the conversion from legacy device profiles to the newer profile
information embedded in the image building code, the legacy recipes got
triggered twice with different eval depths, leading to shell syntax errors
when processing certain images.

The double processing was caused by the remaining Image/Build macro in
legacy.mk which serves as main entry point for the new style image build code
in conjunction with the newly introduced LegacyDevice/* macros which caused
the legacy image build fallback code to kick in.

In order to fix the issue, rework all legacy macros to work under the legacy
image build wrapper and remove the Image/Build macro of legacy.mk to prevent
legacy profiles getting executed in the context of the new build code.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>