build: Expand build dirs in Makefile.buildkit
build: Expand build dirs in Makefile.buildkit

Remove support for user-defined build dir paths when using Docker
buildkit. This was dangerous as there was no easy way to make sure the
user supplied paths were pointing to directories that can be safely
deleted and overwritten.

Expand the BUILD_DIR and DOCKER_BUILD_DIR in Makefile.buildkit to make
it easier to understand and to protect against accidental damage when
BUILD_DIR = '.', which would mess up user's main git repo.

Fixes: #11513
Signed-off-by: Jarno Rajahalme <0f1ab0ac7dffd9db21aa539af2fd4bb04abc3ad4@covalent.io>
tgraf/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,michi-covalent/cilium,tgraf/cilium,cilium-team/cilium,tklauser/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium,cilium/cilium,tklauser/cilium,tklauser/cilium,cilium/cilium,cilium-team/cilium,tklauser/cilium,michi-covalent/cilium,michi-covalent/cilium