feat: make it possible to enable plugins with a build env variable

docs/plugins.md

@@ -127,6 +127,13 @@ To preload a go-ipfs plugin:
 go-ipfs$ make build
 ```
 
+You can also preload an in-tree but disabled-by-default plugin by adding it to
+the IPFS_PLUGINS variable. For example, to enable plugins foo, bar, and baz:
+
+```bash
+go-ipfs$ make build IPFS_PLUGINS="foo bar baz"
+```
+
 ## Creating A Plugin
 
 To create your own out-of-tree plugin, use the [example

plugin/loader/Rules.mk

 include mk/header.mk
 
+IPFS_PLUGINS ?= 
+export IPFS_PLUGINS
+
 $(d)/preload.go: d:=$(d)
-$(d)/preload.go: $(d)/preload_list $(d)/preload.sh
+$(d)/preload.go: $(d)/preload_list $(d)/preload.sh ALWAYS
 	$(d)/preload.sh > $@
 	go fmt $@ >/dev/null
 

plugin/loader/preload.sh

@@ -4,6 +4,11 @@ DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 
 to_preload() {
 	awk 'NF' "$DIR/preload_list" | sed '/^#/d'
+  if [[ -n "$IPFS_PLUGINS" ]]; then
+      for plugin in $IPFS_PLUGINS; do
+          echo "$plugin github.com/ipfs/go-ipfs/plugin/plugins/$plugin *"
+      done
+  fi
 }
 
 cat <<EOL