pkg: u8g2: add u8g2 graphics library.

pkg/u8g2/Makefile

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+PKG_NAME=u8g2
+PKG_URL=https://github.com/olikraus/u8g2
+PKG_VERSION=94dacdb84e06a6088b9d17a1e7ba009dbd3618be
+
+.PHONY: all
+
+all: git-download
+	"$(MAKE)" -C $(PKG_BUILDDIR)
+
+include $(RIOTBASE)/pkg/pkg.mk

pkg/u8g2/Makefile.include

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+INCLUDES += -I$(BINDIRBASE)/pkg/$(BOARD)/u8g2/csrc
+
+# Link SDL if enabled.
+ifneq (,$(filter u8g2_sdl,$(USEMODULE)))
+  LINKFLAGS += `sdl-config --libs`
+endif

pkg/u8g2/README.md

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+# U8g2
+
+## Introduction
+[U8g2](https://github.com/olikraus/u8g2) is a monochrome graphics library for LCDs and OLEDs. It contains both drivers and high-level drawing routines.
+
+The library is originally written for Arduino's, but it runs fine on other platforms if the right drivers are available.
+
+## Usage
+Just put `USEPKG += u8g2` in your Makefile and `#include "u8g.h"` to your code.
+
+## API
+This package patches the original source to add an interface for RIOT-OS peripherals and removing most of the device/platform specific code.
+
+The following two interfaces add add support for the included drivers via I2C and SPI peripherals:
+
+* `u8g_com_riotos_hw_spi_init` — Interface for U8g2 included SPI displays.
+* `u8g_com_riotos_ssd_i2c_init` — Interface for U8g2 included I2C SSD displays.
+
+In addition, the following three drivers are general-purpose interfaces to write your own display driver:
+
+* `u8g_com_riotos_i2c_init` — Generic I2C display driver interface.
+* `u8g_com_riotos_spi_init` — Generic SPI display driver interface.
+* `u8g_com_riotos_init` — General-purpose interface that accepts a void pointer argument.
+
+For targets without an I2C or SPI, the following two interfaces emulate a display:
+
+* `u8g_dev_riotos_stdout_init` — Virtual display via stdout.
+* `u8g_dev_riotos_stdout_ansi_init` — Virtual display via stdout using ANSI control characters.