boards/hifive1b: Clean up flashing parameters

- OpenOCD and J-Link flash configuration can be provided
  unconditionally, as those variables do not conflict with any others
- Only use J-Link as default flasher if actually installed

boards/hifive1b/Makefile.include

@@ -1,16 +1,22 @@
-# Set default programmer as jlink
-PROGRAMMER ?= jlink
 PROGRAMMERS_SUPPORTED += jlink openocd
 
-ifeq (openocd,$(PROGRAMMER))
-  OPENOCD_DEBUG_ADAPTER = jlink
-  OPENOCD_TRANSPORT = jtag
-  OPENOCD_PRE_FLASH_CMDS += "-c flash protect 0 1 last off"
-else ifeq (jlink,$(PROGRAMMER))
-  # setup JLink for flashing
-  JLINK_DEVICE = FE310
-  JLINK_IF = JTAG
-  FLASH_ADDR = 0x20010000
+# OpenOCD parameters
+OPENOCD_DEBUG_ADAPTER := jlink
+OPENOCD_TRANSPORT := jtag
+OPENOCD_PRE_FLASH_CMDS += "-c flash protect 0 1 last off"
+
+# setup JLink for flashing
+JLINK_DEVICE := FE310
+JLINK_IF := JTAG
+FLASH_ADDR := 0x20010000
+
+# keep name of `JLINK` in sync with script jlink.sh in $(RIOTTOOLS)/jlink
+# Default to J-Link as programmer when installed, otherwise go for OpenOCD
+JLINK ?= JLinkExe
+ifneq (,$(shell command -v $(JLINK)))
+  PROGRAMMER ?= jlink
+else
+  PROGRAMMER ?= openocd
 endif
 
 TESTRUNNER_RESET_DELAY = 1