Makefile: Allow external Makefile.build

The current method of building libraries is to iterate over
each library's source files and invoke a file type specific
build rule. For libgo this is insufficient as we have to do
more complex operations for dependency resolution
before starting the actual build. This commit allows libraries
to supply an own `Makefile.build` that enables custom
build mechanics.

Co-authored-by: Marc Rittinghaus <marc.rittinghaus@unikraft.io>
Signed-off-by: Marc Rittinghaus <marc.rittinghaus@unikraft.io>
Signed-off-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Reviewed-by: Stefan Jumarea <stefanjumarea02@gmail.com>
Reviewed-by: Radu Nichita <radunichita99@gmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: unikraft#1005

support/build/Makefile.build

@@ -38,6 +38,11 @@ $(call verbose_info,Warning: '$(L)' has no sources or object files$(comma) ignor
 )
 endif
 
+$(foreach _M,$(wildcard $(addsuffix Makefile.build,\
+	   $(CONFIG_UK_BASE)/lib/*/ $(CONFIG_UK_BASE)/plat/*/ \
+	   $(addsuffix /,$(ELIB_DIR)) $(APP_DIR)/)), \
+		$(eval $(call verbose_include,$(_M))) \
+)
 
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