CONTRIB: move src/da.c and contrib/deviceatlas to addons/deviceatlas

Both the source file and the dummy library are now at the same place.
Maybe the build howto could be moved there as well to make things even
cleaner.

The Makefile, MAINTAINERS, doc, github build matrix, coverity checks
and travis CI's build were updated.
diff --git a/Makefile b/Makefile
index a9e8b49..b0cfe23 100644
--- a/Makefile
+++ b/Makefile
@@ -640,7 +640,7 @@
 OPTIONS_OBJS	+= $(DEVICEATLAS_LIB)/json.o
 OPTIONS_OBJS	+= $(DEVICEATLAS_LIB)/dac.o
 endif
-OPTIONS_OBJS	+= src/da.o
+OPTIONS_OBJS	+= addons/deviceatlas/da.o
 OPTIONS_CFLAGS += $(if $(DEVICEATLAS_INC),-I$(DEVICEATLAS_INC))
 endif
 
@@ -1005,6 +1005,7 @@
 	$(Q)rm -f {admin,dev,contrib}/*/*.[oas] {admin,dev,contrib}/*/*/*.[oas] {admin,dev,contrib}/*/*/*/*.[oas]
 	$(Q)rm -f addons/promex/*.[oas]
 	$(Q)rm -f addons/51degrees/*.[oas] addons/51degrees/dummy/*.[oas] addons/51degrees/dummy/*/*.[oas]
+	$(Q)rm -f addons/deviceatlas/*.[oas] addons/deviceatlas/dummy/*.[oas]
 	$(Q)rm -f admin/iprange/iprange admin/iprange/ip6range admin/halog/halog
 	$(Q)rm -f dev/flags/flags dev/poll/poll dev/tcploop/tcploop
 	$(Q)rm -f dev/hpack/decode dev/hpack/gen-enc dev/hpack/gen-rht