tests/riotboot: use FLASHFILE for the generated file

FLASHFILE is now a generated file when doing `make all`.
This prepares also for when flashers will use `FLASHFILE` as a file to
be flashed.

It currently still needs the hack below for openocd and edbg.

This also fixes the issue when building 'riotboot' in docker that was
being built with the host toolchain.

tests/riotboot/Makefile

@@ -18,17 +18,15 @@ DEVELHELP ?= 1
 # Change this to 0 show compiler invocation lines by default:
 QUIET ?= 1
 
-all: riotboot
+# Target 'all' will generate the combined file directly.
+# It also makes 'flash' and 'flash-only' work without specific command.
+FLASHFILE = $(RIOTBOOT_COMBINED_BIN)
 
 include ../Makefile.tests_common
 include $(RIOTBASE)/Makefile.include
 
-# Make 'flash' and 'flash-only' work without specific command.
-# This is currently hacky as there is no way of specifiying a FLASHFILE
-all: riotboot/combined-slot0
+# This is currently hacky as the flasher are not using 'FLASHFILE'
 # openocd
-ELFFILE = $(RIOTBOOT_COMBINED_BIN)
+ELFFILE = $(FLASHFILE)
 # edbg
-HEXFILE = $(RIOTBOOT_COMBINED_BIN)
-# murdock uses ':=' to get the flashfile variable so should also be overwritten
-FLASHFILE = $(RIOTBOOT_COMBINED_BIN)
+HEXFILE = $(FLASHFILE)