Makefile: allow overriding the config file

Change-Id: If077e9c0e06d7b081b5254bcded77ee65bcd8e5d
---
M Makefile
1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index 2a14fe7..4482b2c 100644
--- a/Makefile
+++ b/Makefile
@@ -1,3 +1,5 @@
+CONFIG ?= config/sys.config
+
 all: build
 
 GEN_FILES = include/S1AP-*.hrl src/S1AP-*.erl
@@ -10,10 +12,10 @@
 	rebar3 escriptize
 
 run: build
-	ERL_FLAGS='-config config/sys.config' _build/default/bin/osmo-s1gw
+	ERL_FLAGS='-config $(CONFIG)' _build/default/bin/osmo-s1gw
 
 shell: build
-	rebar3 shell --config ./config/sys.config
+	rebar3 shell --config $(CONFIG)
 
 check: $(GEN_FILES)
 	rebar3 eunit

