Only support building a single board at a time

As a follow up to c461e20df3 ("make: Remove version from build output
path"), only support building a single version of a single board by
default. The build directory can still be specified by passing `BUILD`.

Fixes running the `clean` target when `BUILD` is a custom value or
`BOARD` isn't set.

Fixes: b03c960b4f ("make: Reduce build output")
Signed-off-by: Tim Crawford <tcrawford@system76.com>

Makefile

@@ -10,6 +10,9 @@ ifneq ($(VERBOSE),1)
 MAKEFLAGS += -s
 endif
 
+# Set build directory
+BUILD = build
+
 # Parameter for current board
 ifeq ($(BOARD),)
 all:
@@ -21,10 +24,6 @@ DATE=$(shell git show --format="%cs" --no-patch --no-show-signature)
 REV=$(shell git describe --abbrev=7 --always --dirty)
 VERSION?=$(DATE)_$(REV)
 
-# Set build directory
-obj = build
-BUILD = $(obj)/$(BOARD)
-
 # Default target - build the board's EC firmware
 all: $(BUILD)/ec.rom
 	$(info Built $(VERSION) for $(BOARD))
@@ -72,7 +71,7 @@ docs:
 
 .PHONY: clean
 clean:
-	rm -rf $(obj)
+	rm -rf $(BUILD)
 
 .PHONY: fmt
 fmt: