stm32l4: include missing makefile

Fixes: b8b97209 stm32l4: basic uart/exti/led/clock demo

which introduced the l4 example, but left out the makefile.include file
and didn't add it to the root build.

Reported at: 2a7e4fac14

examples/stm32/l4/Makefile.include

@@ -0,0 +1,44 @@
+##
+## This file is part of the libopencm3 project.
+##
+## This library is free software: you can redistribute it and/or modify
+## it under the terms of the GNU Lesser General Public License as published by
+## the Free Software Foundation, either version 3 of the License, or
+## (at your option) any later version.
+##
+## This library is distributed in the hope that it will be useful,
+## but WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+## GNU Lesser General Public License for more details.
+##
+## You should have received a copy of the GNU Lesser General Public License
+## along with this library.  If not, see <http://www.gnu.org/licenses/>.
+##
+
+# You should use linker script generation! Specify device!
+ifeq ($(DEVICE),)
+LIBNAME		= opencm3_stm32l4
+DEFS		+= -DSTM32L4
+
+FP_FLAGS	?= -mfloat-abi=hard -mfpu=fpv4-sp-d16
+ARCH_FLAGS	= -mthumb -mcpu=cortex-m4 $(FP_FLAGS)
+endif
+
+################################################################################
+# OpenOCD specific variables
+
+OOCD		?= openocd
+OOCD_INTERFACE	?= stlink-v2-1
+OOCD_TARGET	?= stm32l4x
+
+################################################################################
+# Black Magic Probe specific variables
+# Set the BMP_PORT to a serial port and then BMP is used for flashing
+BMP_PORT	?=
+
+################################################################################
+# texane/stlink specific variables
+#STLINK_PORT	?= :4242
+
+
+include ../../../../rules.mk