pkg: add heatshrink (compression library)

pkg/heatshrink/Makefile

+PKG_NAME=heatshrink
+PKG_URL=https://github.com/atomicobject/heatshrink.git
+PKG_VERSION=7d419e1fa4830d0b919b9b6a91fe2fb786cf3280
+
+.PHONY: all
+
+all: git-download
+	cp Makefile.heatshrink $(PKG_BUILDDIR)/Makefile
+	"$(MAKE)" -C $(PKG_BUILDDIR)
+
+include $(RIOTBASE)/pkg/pkg.mk

pkg/heatshrink/Makefile.heatshrink

+SRC := heatshrink_decoder.c heatshrink_encoder.c
+include $(RIOTBASE)/Makefile.base

pkg/heatshrink/Makefile.include

+CFLAGS += -DHEATSHRINK_DYNAMIC_ALLOC=0
+INCLUDES += -I$(BINDIRBASE)/pkg/$(BOARD)/heatshrink

pkg/heatshrink/README.md

+# Introduction
+
+This package provides a compression library specifically developed for
+memory-constrained devices.  See https://github.com/atomicobject/heatshrink for
+more information.
+
+# License
+
+The library is ISC licensed.