Makefile: Allow user to override libosmocore install prefix

We should probably use PKG_CONFIG to do this in the next step..

Change-Id: Ie1eda223da3e22b6b1eea73cfcca499cf3ba7940
diff --git a/sysmoOCTSIM/gcc/Makefile b/sysmoOCTSIM/gcc/Makefile
index 01088cc..282a146 100644
--- a/sysmoOCTSIM/gcc/Makefile
+++ b/sysmoOCTSIM/gcc/Makefile
@@ -2,8 +2,9 @@
 # Manually edited. Do not overwrite with Automatically-generated file.
 ################################################################################
 
-EXTRA_LIBS=/usr/local/arm-none-eabi/lib/libosmocore.a
-EXTRA_CFLAGS=-I/usr/local/arm-none-eabi/include -I../../ccid
+SYSTEM_PREFIX:=/usr/local/arm-none-eabi
+EXTRA_LIBS=$(SYSTEM_PREFIX)/lib/libosmocore.a
+EXTRA_CFLAGS=-I$(SYSTEM_PREFIX)/include -I../../ccid
 CROSS_COMPILE= arm-none-eabi-
 
 CFLAGS_CPU=-D__SAME54N19A__ -mcpu=cortex-m4 -mfloat-abi=softfp -mfpu=fpv4-sp-d16