Makefile: Factor out coverage generation
Makefile: Factor out coverage generation

Create a new make target for coverage HTML generation, so it can be
reused by other make targets.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

tklauser/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,cilium/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,cilium-team/cilium,cilium-team/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium