Add Makefile command line variables to enable docker and podman

Instead of having to edit the Makefile, we can now do:

make DOCKER=1
make PODMAN=1

Signed-off-by: Anton Blanchard <anton@linux.ibm.com>

Makefile

@@ -1,20 +1,29 @@
-GHDL=ghdl
+GHDL ?= ghdl
 GHDLFLAGS=--std=08 --work=unisim
 CFLAGS=-O2 -Wall
 
 # We need a version of GHDL built with either the LLVM or gcc backend.
 # Fedora provides this, but other distros may not. Another option is to use
 # the Docker image.
-#
-# Uncomment one of these to build with Docker or podman
-#DOCKER=docker
-DOCKER=podman
-#
-# Uncomment these lines to build with Docker/podman
+DOCKER ?= 0
+PODMAN ?= 0
+
+ifeq ($(DOCKER), 1)
+DOCKERBIN=docker
+USE_DOCKER=1
+endif
+
+ifeq ($(PODMAN), 1)
+DOCKERBIN=podman
+USE_DOCKER=1
+endif
+
+ifeq ($(USE_DOCKER), 1)
 PWD = $(shell pwd)
 DOCKERARGS = run --rm -v $(PWD):/src:z -w /src
-GHDL = $(DOCKER) $(DOCKERARGS) ghdl/ghdl:buster-llvm-7 ghdl
-CC = $(DOCKER) $(DOCKERARGS) ghdl/ghdl:buster-llvm-7 gcc
+GHDL = $(DOCKERBIN) $(DOCKERARGS) ghdl/ghdl:buster-llvm-7 ghdl
+CC = $(DOCKERBIN) $(DOCKERARGS) ghdl/ghdl:buster-llvm-7 gcc
+endif
 
 all = core_tb icache_tb dcache_tb multiply_tb dmi_dtm_tb divider_tb \
 	rotator_tb countzero_tb wishbone_bram_tb soc_reset_tb

README.md

@@ -37,7 +37,7 @@ cd ../../../
   appears not to). ghdl with the LLVM backend is likely easier to build.
 
   If building ghdl from scratch is too much for you, the microwatt Makefile
-  supports using Docker or podman images. Read through the Makefile for details.
+  supports using Docker or Podman.
 
 - Next build microwatt:
 
@@ -45,6 +45,17 @@ cd ../../../
 git clone https://github.com/antonblanchard/microwatt
 cd microwatt
 make
+```
+
+   To build using Docker:
+```
+make DOCKER=1
+```
+
+   and to build using Podman:
+
+```
+make PODMAN=1
 ```
 
 - Link in the micropython image: