Makefile: fix d_lcd dependency on images

d_lcd.o needs normal_font.xbm to be generated. Express this in the Makefile
dependencies for correct build.

Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
rules.mk: Allow to seed some variables from env

Append to INCLUDES, CFLAGS and LDFLAGS instead of assigning, to allows
"seeding" those variables from env.

This allows to specify additional include paths for example, or additional
flags.

Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>