Build: fix build conf concept

A build variable is evaluated like:
1.) if the var is already defined in the shell env, use that.
2.) if the var is defined in the the build.conf, use that.
3. fallback to using the var default in Makefile.conf

Makefile.conf

@@ -1,4 +1,5 @@
 HERE:=$(dir $(lastword $(MAKEFILE_LIST)))
+-include $(HERE)build.conf
 
 # Project Name
 TARGET?=perkons
@@ -7,21 +8,18 @@ TARGET?=perkons
 # common configurations:
 # use DEBUG = 1 and OPT = -Og for debugging
 # or DEBUG = 0 and OPT = -O3 for performance
-DEBUG=0
-OPT=-O3
+DEBUG?=0
+OPT?=-O3
 
 # (optional) Includes FatFS source files within project.
 #USE_FATFS = 1
 
 # Relative to dir 'src'
-LIBDAISY_DIR=../../heck_libDaisy
-DAISYSP_DIR=../../heck_DaisySP
+LIBDAISY_DIR?=../../heck_libDaisy
+DAISYSP_DIR?=../../heck_DaisySP
 
 CXXFLAGS+=-Wall -Wno-unused -Wno-reorder-ctor -Wno-switch
 
-######### Overrides from build.conf #########
--include $(HERE)build.conf
-
 ifneq (,$(findstring g++,$(CXX)))
     CXXFLAGS+=-fdiagnostics-color=always
 else ifneq (,$(findstring clang,$(CXX)))

build.conf.example

@@ -3,10 +3,10 @@
 
 # The program to compile and flash
 # the name of the main cxx file without the prefix 'main_'
-TARGET=perkons
+TARGET?=perkons
 
-DEBUG=0
+DEBUG?=0
 
 # Daisy dependencies Relative to dir 'src'
-LIBDAISY_DIR=../../heck_libDaisy
-DAISYSP_DIR=../../heck_DaisySP
+LIBDAISY_DIR?=../../heck_libDaisy
+DAISYSP_DIR?=../../heck_DaisySP