gateware: Allow override of NO2BUILD_DIR from command line

Signed-off-by: Sylvain Munaut <tnt@246tNt.com>
diff --git a/gateware/icE1usb-proto/Makefile b/gateware/icE1usb-proto/Makefile
index bd54a0f..d17a540 100644
--- a/gateware/icE1usb-proto/Makefile
+++ b/gateware/icE1usb-proto/Makefile
@@ -35,7 +35,7 @@
 NEXTPNR_ARGS = --pre-pack data/clocks.py --seed 4
 
 # Include default rules
-NO2BUILD_DIR := ../build
+NO2BUILD_DIR ?= ../build
 include $(NO2BUILD_DIR)/project-rules.mk
 
 # Custom rules