diff --git a/Makefile.ac b/Makefile.ac
index a0ca229775..f49755c7a4 100644
--- a/Makefile.ac
+++ b/Makefile.ac
@@ -33,9 +33,8 @@ ACINCLUDE = $(PAPARAZZI_HOME)/var/$(AIRCRAFT)
 AIRCRAFT_CONF_DIR = $(ACINCLUDE)/conf
 AC_GENERATED = $(ACINCLUDE)/generated
 
-ifndef PERIODIC_FREQ
-PERIODIC_FREQ = 60
-endif
+# periodic frequency defaults to 60Hz
+PERIODIC_FREQ ?= 60
 
 AIRFRAME_H=$(AC_GENERATED)/airframe.h
 PERIODIC_H=$(AC_GENERATED)/periodic_telemetry.h
diff --git a/conf/Makefile.lpc21 b/conf/Makefile.lpc21
index 109e8f2e98..a1b8bc298d 100644
--- a/conf/Makefile.lpc21
+++ b/conf/Makefile.lpc21
@@ -126,9 +126,7 @@ LDFLAGS +=-T$(LDSCRIPT)
 
 # Settings and variables:
 LPC21ISP = lpc21isp
-ifndef LPC21ISP_PORT
-LPC21ISP_PORT = /dev/ttyS0
-endif
+LPC21ISP_PORT ?= /dev/ttyS0
 LPC21ISP_FLASHFILE = $(OBJDIR)/$(TARGET).hex
 # verbose output:
 #LPC21ISP_DEBUG = -debug
diff --git a/conf/Makefile.stm32 b/conf/Makefile.stm32
index b57476046c..877b2c11a1 100644
--- a/conf/Makefile.stm32
+++ b/conf/Makefile.stm32
@@ -45,9 +45,7 @@ OPT   = s
 #OPT   = 0
 
 
-ifndef $(PYTHON)
-PYTHON = $(shell which python)
-endif
+PYTHON ?= $(shell which python)
 
 ifneq ($(BOARD_SERIAL),)
 OOCD_OPTIONS = -c "ft2232_serial $(BOARD_SERIAL)"
@@ -266,11 +264,10 @@ upload: $(OBJDIR)/$(TARGET).hex
 		-c shutdown
 else
 # jtag via BMP
-  ifndef $(BMP_UPLOAD_SCRIPT)
-    BMP_UPLOAD_SCRIPT = $(PAPARAZZI_SRC)/conf/boards/upload_scripts/bmp_jtag_flash.scr
-  endif
+BMP_UPLOAD_SCRIPT ?= $(PAPARAZZI_SRC)/conf/boards/upload_scripts/bmp_jtag_flash.scr
 
 upload: $(OBJDIR)/$(TARGET).elf
+	@echo "Assuming luftboot bootloader: $(ASSUMING_LUFTBOOT)"
 	@echo "Using Black Magic Probe with JTAG"
 	@echo "Using GDB = $(GDB)"
 	@echo "  BMP\t$<"
@@ -284,10 +281,9 @@ endif
 else ifeq ($(FLASH_MODE),SWD)
 # only works if BMP_PORT is defined
 ifneq ($(BMP_PORT),)
-  ifndef $(BMP_UPLOAD_SCRIPT)
-    BMP_UPLOAD_SCRIPT = $(PAPARAZZI_SRC)/conf/boards/upload_scripts/bmp_swd_flash.scr
-  endif
+BMP_UPLOAD_SCRIPT ?= $(PAPARAZZI_SRC)/conf/boards/upload_scripts/bmp_swd_flash.scr
 upload: $(OBJDIR)/$(TARGET).elf
+	@echo "Assuming luftboot bootloader: $(ASSUMING_LUFTBOOT)"
 	@echo "Using Black Magic Probe with SWD"
 	@echo "Using GDB = $(GDB)"
 	@echo "  BMP\t$<"