Makefile.base: use thin static archives.

Normal, or thick archives contain a copy of the object code. Thin
archives, on the contrary, are just an index to the .o files.

This patch does two things:

1. Change ARFLAGS to enable the "T" options.
2. Call AR with all relative paths.

The second step is necessary because the build system handles all
absolute paths. If the index in the thin archive contains absolute
paths, archives created in docker are no usable outside, and moving
the objects breaks the archive.

If all arguments to AR are relative, the resulting archive contains
filenames *relative to the .a file* and nothing should break as long
as the relative location of the .a and .o remains unchanged.

Compilation time is unchanged, but disc usage is reduced by approximately
50%. These are the result of a full RIOT build:

| Thin Archive   |  no     |  yes  | Savings (%) |
| -------------- | ------: | ----: | ----------- |
| pkg (10e6 KiB) | 1 790   | 905   | 49%         |
| Non pkg        |    71   |  71   | 1%          |
| Total          | 1 812   | 976   | 46 %        |

Makefile.base

@@ -67,10 +67,12 @@ $(BINDIR)/$(MODULE)/:
 
 $(BINDIR)/$(MODULE).a $(OBJ): | $(BINDIR)/$(MODULE)/
 
+relpath = $(shell realpath --relative-to=$(abspath .) $(1))
+
 $(BINDIR)/$(MODULE).a: $(OBJ) | $(DIRS:%=ALL--%)
 	@# Recreate archive to cleanup deleted/non selected source files objects
 	$(Q)$(RM) $@
-	$(Q)$(AR) $(ARFLAGS) $@ $^
+	$(Q)$(AR) $(ARFLAGS) $(foreach f,$@ $^,$(call relpath,$f))
 
 CXXFLAGS = $(filter-out $(CXXUWFLAGS), $(CFLAGS)) $(CXXEXFLAGS)
 CCASFLAGS = $(filter-out $(CCASUWFLAGS), $(CFLAGS)) $(CCASEXFLAGS)

makefiles/cflags.inc.mk

@@ -65,5 +65,5 @@ CFLAGS += $(filter-out $(OPTIONAL_CFLAGS_BLACKLIST),$(OPTIONAL_CFLAGS))
 # Default ARFLAGS for platforms which do not specify it.
 # Note: make by default provides ARFLAGS=rv which we want to override
 ifeq ($(origin ARFLAGS),default)
-  ARFLAGS = rcs
+  ARFLAGS = rcTs
 endif