esp32/boards/GENERIC_D2WD: Build with -Os optimisation.

This board has only 2MiB of flash so the build needs to be reduced in size
to fit.  Commit 549448e8 made all boards
build with -O2 by default (for performance) so this overrides that default.
Signed-off-by: Damien George <damien@micropython.org>

ports/esp32/boards/GENERIC_D2WD/sdkconfig.board

+# Optimise using -Os to reduce size
+CONFIG_COMPILER_OPTIMIZATION_SIZE=y
+CONFIG_COMPILER_OPTIMIZATION_PERF=n
+
 CONFIG_ESPTOOLPY_FLASHMODE_DIO=y
 CONFIG_ESPTOOLPY_FLASHFREQ_40M=y
 CONFIG_ESPTOOLPY_FLASHSIZE_2MB=y