Add FPGA_TARGET=ECP5-EVN make option for synthesis build

This allows these targets
  FPGA_TARGET=ORANGE-CRAB make microwatt.bit
  FPGA_TARGET=ECP5-EVN make microwatt.bit
Default is ORANGE-CRAB as before

ECP5-EVN is tested on real hardware. The console only works at 38400 so
needs this in console.c and a recompile of hello_world to work:

  -#define UART_FREQ 115200
  +#define UART_FREQ 38400

With this 'FPGA_TARGET=ECP5-EVN make prog' works on the ECP5 dev board.

Signed-off-by: Michael Neuling <mikey@neuling.org>

Makefile

@@ -143,7 +143,10 @@ RAM_INIT_FILE=hello_world/hello_world.hex
 #MEMORY_SIZE=393216
 #RAM_INIT_FILE=micropython/firmware.hex
 
+FPGA_TARGET ?= ORANGE-CRAB
+
 # OrangeCrab with ECP85
+ifeq ($(FPGA_TARGET), ORANGE-CRAB)
 RESET_LOW=true
 CLK_INPUT=50000000
 CLK_FREQUENCY=50000000
@@ -152,16 +155,19 @@ PACKAGE=CSFBGA285
 NEXTPNR_FLAGS=--um5g-85k --freq 50
 OPENOCD_JTAG_CONFIG=openocd/olimex-arm-usb-tiny-h.cfg
 OPENOCD_DEVICE_CONFIG=openocd/LFE5UM5G-85F.cfg
+endif
 
 # ECP5-EVN
-#RESET_LOW=true
-#CLK_INPUT=12000000
-#CLK_FREQUENCY=12000000
-#LPF=constraints/ecp5-evn.lpf
-#PACKAGE=CABGA381
-#NEXTPNR_FLAGS=--um5g-85k --freq 12
-#OPENOCD_JTAG_CONFIG=openocd/ecp5-evn.cfg
-#OPENOCD_DEVICE_CONFIG=openocd/LFE5UM5G-85F.cfg
+ifeq ($(FPGA_TARGET), ECP5-EVN)
+RESET_LOW=true
+CLK_INPUT=12000000
+CLK_FREQUENCY=12000000
+LPF=constraints/ecp5-evn.lpf
+PACKAGE=CABGA381
+NEXTPNR_FLAGS=--um5g-85k --freq 12
+OPENOCD_JTAG_CONFIG=openocd/ecp5-evn.cfg
+OPENOCD_DEVICE_CONFIG=openocd/LFE5UM5G-85F.cfg
+endif
 
 GHDL_IMAGE_GENERICS=-gMEMORY_SIZE=$(MEMORY_SIZE) -gRAM_INIT_FILE=$(RAM_INIT_FILE) \
 	-gRESET_LOW=$(RESET_LOW) -gCLK_INPUT=$(CLK_INPUT) -gCLK_FREQUENCY=$(CLK_FREQUENCY)