Add INCLUDE command

Squashed commit from #33.

* Add INCLUDE command to include other Pifiles easily in our script without using sh'isms.

* Fix typo

* Move INCLUDE to 00-commands.sh so it's available in all stages.
Add description of INCLUDE command to README.md

* Fix typo

* Incorporating suggestions by @oxzi

* stages/00-commands.sh: fix shellcheck warning

* INCLUDE examples

Co-authored-by: Alvar Penning <post@0x21.biz>

README.md

@@ -106,6 +106,12 @@ default to *rpi.img* in the source file's directory.
 `INPLACE` does not create a copy of the image, but performs all further
 operations on the given image. This is an alternative to `FROM` and `TO`.
 
+#### `INCLUDE [PATH_TO_PIFILE]`
+`INCLUDE` includes the provided Pifile in the current one for modularity and 
+re-use. The included file _has_ to have a `.Pifile` extension which need not be 
+specified.
+
+
 #### `PUMP [SIZE]`
 `PUMP` increases the image's size about the given amount (suffixes K, M, G are allowed).
 

examples/Module-Hello.Pifile

@@ -0,0 +1,3 @@
+# This is a simple example for an `INCLUDE`able module.
+
+RUN echo "Included echo!"

examples/RPi-OpenWRT.Pifile

@@ -21,3 +21,7 @@ RUN hello
 # test if INSTALL works
 INSTALL "README.md" "/tmp/"
 RUN head -n1 "/tmp/README.md"
+
+# test an INCLUDE - both with and without the .Pifile extension
+INCLUDE examples/Module-Hello.Pifile
+INCLUDE examples/Module-Hello

stages/00-commands.sh

@@ -18,6 +18,15 @@ post_stage() {
   :
 }
 
+# INCLUDE sources the provided Pifile
+#
+# Usage: INCLUDE path/to/pifile[.Pifile]
+INCLUDE() {
+  filename="${1%.*}"
+  # shellcheck disable=SC1090
+  source "${filename}.Pifile"
+}
+
 # Stage 1x
 FROM() {
   :
@@ -31,6 +40,7 @@ INPLACE() {
   :
 }
 
+
 # Stage 2x
 PUMP() {
   :