Allow custom stages

STAGE_LIST can be specified in the config file to run additional build stages.

README.md

@@ -89,6 +89,10 @@ The following environment variables are supported:
 
    Setting to `1` will enable ssh server for remote log in. Note that if you are using a common password such as the defaults there is a high risk of attackers taking over you RaspberryPi.
 
+ * `STAGE_LIST` (Default: `stage*`)
+
+    If set, then instead of working through the numeric stages in order, this list will be followed. For example setting to `stage0 stage1 mystage stage2` will run the contents of `mystage` before stage2. An absolute or relative path can be given for stages outside the pi-gen directory.
+
 A simple example for building Raspbian:
 
 ```bash

build.sh

@@ -195,7 +195,10 @@ dependencies_check "${BASE_DIR}/depends"
 mkdir -p "${WORK_DIR}"
 log "Begin ${BASE_DIR}"
 
-for STAGE_DIR in "${BASE_DIR}/stage"*; do
+STAGE_LIST=${STAGE_LIST:-${BASE_DIR}/stage*}
+
+for STAGE_DIR_ in $STAGE_LIST; do
+	STAGE_DIR=`realpath "${STAGE_DIR_}"`
 	run_stage
 done