pkg: add support for Cayenne LPP format

pkg/cayenne-lpp/Makefile

+PKG_NAME=cayenne-lpp
+PKG_URL=https://github.com/aabadie/cayenne-lpp
+PKG_VERSION=ad3aa26043e221eda28ac3da2484e1ef84986442
+PKG_LICENSE=LGPLv2.1
+
+.PHONY: all
+
+all: git-download
+	"$(MAKE)" -C $(PKG_BUILDDIR) -f $(CURDIR)/Makefile.$(PKG_NAME)
+
+include $(RIOTBASE)/pkg/pkg.mk

pkg/cayenne-lpp/Makefile.cayenne-lpp

+MODULE = cayenne-lpp
+
+include $(RIOTBASE)/Makefile.base

pkg/cayenne-lpp/Makefile.include

+INCLUDES += -I$(PKGDIRBASE)/cayenne-lpp

pkg/cayenne-lpp/doc.txt

+/**
+ * @defgroup pkg_cayenne-lpp Cayenne Low Power Payload (LPP)
+ * @ingroup  pkg
+ * @ingroup  sys
+ * @brief    Provides a RIOT support for Cayenne LPP format
+ *
+ * The Cayenne Low Power Payload (LPP) provides a convenient and easy way to
+ * send data over LPWAN networks such as LoRaWAN.
+ *
+ * For more details on the format, see
+ * https://mydevices.com/cayenne/docs_stage/lora/#lora-cayenne-low-power-payload
+ *
+ * @see      https://github.com/aabadie/cayenne-lpp
+ */