iprove user guidance

distinguish in output between "PoE module" and Raspberry Pi CM 4

rpi/write_to

@@ -12,7 +12,12 @@ IMAGES_HOST = 'l_samenv@linse-c'
 IMAGES_DIR = 'boxes/%s_images'
 IMAGES_PAT = '*.lz4'
 
-EMMC = {'rpi', 'ionopimax'}  # write over USB, needs rpiboot. else SD card is assumed
+# the following box types are flashed over USB, and needs rpiboot.
+# else SD card is assumed
+EMMC = {
+    'rpi': 'Raspberry Pi with compute module',
+    'ionopimax': 'compute module on PoE device',
+}
 
 
 def prt(text):
@@ -205,10 +210,11 @@ class Darwin(HostSystem):
         return proposed
 
     def write_image(self):
-        if self.kind in EMMC:
+        emmc_descr = EMMC.get(self.kind)
+        if emmc_descr:
             print("\nmake sure microUSB 'SLAVE' is connected to this computer")
             print("and microUSB 'POWER' is connected to a USB power supply\n")
-            prt('try to find CM (compute module on PoE device) ...')
+            prt(f'try to find CM ({emmc_descr}) ...')
         else:
             pass
         proposed = self.find_raspi()