stm32/mboot/Makefile: Revert change to BOARD_DIR that removed abspath.

This reverts the change from ce2058685b.

Without abspath, the build artefacts (object files) for boards with source
files are placed outside the build directory, because the BOARD_DIR
variable starts with "..".  For the list of source files added to SRC_C,
none of them can start with "..".  The usual fix for that would be to make
the files relative to the top of the MicroPython repo (because of the vpath
rule), eg ports/stm32/boards/$(BOARD).  But then the $(wildcard ...)
pattern won't find files in this directory.

So abspath is necessary, although it will prevent building when there is a
space in the path.  A better solution for spaces needs to be found.

Signed-off-by: Damien George <damien@micropython.org>

ports/stm32/mboot/Makefile

@@ -6,7 +6,7 @@ BOARD ?= $(notdir $(BOARD_DIR:/=))
 else
 # If not given on the command line, then default to PYBV10.
 BOARD ?= PYBV10
-BOARD_DIR ?= ../boards/$(BOARD)
+BOARD_DIR ?= $(abspath ../boards/$(BOARD))
 endif
 
 # If the build directory is not given, make it reflect the board name.