Makefile: Rename default target in proxy Makefile

The default target in Makefile.root is called 'default', while the one
in the proxy Makefile is called 'all'. This was causing unexpected
behaviour.

Test: make
Change-Id: I69103036d62721c739e3a3b6ea22367000668bc8
diff --git a/Makefile b/Makefile
index f10f842..8f0cca7 100644
--- a/Makefile
+++ b/Makefile
@@ -24,7 +24,8 @@
 .PHONY: all %
 
 # At least one target must be defined for `make` (no arguments) to succeed.
-all:
+# The name must match the default rule in $MAKEFILE.
+default:
 	$(MAKE) -f $(MAKEFILE) $@
 
 # Catch-all target that propagates the call to the root Makefile.