Makefile: Cache all macros that may be configured
Makefile: Cache all macros that may be configured

Evaluate and cache all of these macros which are referred to from make
targets multiple times, then conditionally reference the cached version
of the macro depending on whether a developer provides the value via
commandline or env variable.

Greatly speeds up running things like unit-tests.

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

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