boards/samd21-arduino-bootloader: increase preflash delay

The USB bootloader can take a while to be ready / Linux takes a while
to set up the interface.
1s is right at the edge, leading to races and often failed flashing.

Bump the delay to 2s for a wider safety margin, with this flashing is
a lot more reliable.

boards/common/samd21-arduino-bootloader/Makefile.include

@@ -12,4 +12,5 @@ ifeq ($(PROGRAMMER),bossa)
     ROM_OFFSET ?= 0x2000
   endif
   BOSSA_ARDUINO_PREFLASH = yes
+  PREFLASH_DELAY ?= 2
 endif