Build: add propper build configuration concept

Makefile.conf

@@ -3,8 +3,23 @@
 # This file may be used under the terms of the GNU General Public License version 3
 # see LICENSE
 
-BUILD_CONF?=./build.conf
+# BUILD_CONF must be specified relative to the repo root or must be an absolute path
+# BUILD_CONF defaults to ./build.conf
+REPO_ROOT_REL:=$(dir $(lastword $(MAKEFILE_LIST)))
+ifndef BUILD_CONF
+    BUILD_CONF=$(REPO_ROOT_REL)/build.conf
     -include $(BUILD_CONF)
+else
+    BUILD_CONF_EFF=$(BUILD_CONF)
+    ifeq ($(dir $(BUILD_CONF)),./)
+        BUILD_CONF_EFF=$(REPO_ROOT_REL)/$(BUILD_CONF)
+    endif
+    ifeq ($(wildcard $(BUILD_CONF_EFF)),)
+        $(info BUILD_CONF must be specified relative to the repo root or must be an absolute path)
+        $(error file specified using BUILD_CONF ($(BUILD_CONF)) not found)
+    endif
+    include $(BUILD_CONF_EFF)
+endif
 
 TARGET=libpEpCxx11.a