pkg/lua: add download and build of lua as pkg

- download of v5.3.4 of Lua from git
- building using RIOT build system
- patched to remove the need for _times and _link to be
provided to Lua via newlib.

pkg/lua/Makefile

+PKG_NAME=lua
+PKG_URL=https://github.com/lua/lua.git
+PKG_VERSION=e354c6355e7f48e087678ec49e340ca0696725b1
+PKG_LICENSE=MIT
+
+.PHONY: all
+
+all:
+	@cp Makefile.lua $(PKG_BUILDDIR)
+	"$(MAKE)" -C $(PKG_BUILDDIR) -f Makefile.lua
+
+include $(RIOTBASE)/pkg/pkg.mk

pkg/lua/Makefile.include

+INCLUDES += -I$(PKGDIRBASE)/lua

pkg/lua/Makefile.lua

+SRC := $(filter-out lua.c luac.c,$(wildcard *.c))
+
+# This builds for native using POSIX system calls and some extra libraries, and
+# removes a compiler warning that warns against using tmpnam().
+ifeq ($(BOARD),native)
+  CFLAGS += -DLUA_USE_LINUX
+endif
+
+include $(RIOTBASE)/Makefile.base

pkg/lua/doc.txt

+/**
+ * @defgroup pkg_lua Lua ported to RIOT
+ * @ingroup  pkg
+ * @brief    Provides Lua support for RIOT
+ * @see      https://github.com/lua/lua
+ */