build: log time taken by each packages/steps

The idea is to easily get the list of packages taking the most time to build,
and see if we can improve them

v1: Use SetupHostCommand as recommended by John
v2: add space after 'time:', remove useless /usr/bin/time from SetupHostCommand

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>

include/prereq-build.mk

@@ -138,6 +138,10 @@ $(eval $(call SetupHostCommand,bzip2,Please install 'bzip2', \
 $(eval $(call SetupHostCommand,wget,Please install GNU 'wget', \
 	wget --version | grep GNU))
 
+$(eval $(call SetupHostCommand,gtime,Please install GNU 'time', \
+	gtime --version 2>&1 | grep GNU, \
+	time --version 2>&1 | grep GNU))
+
 $(eval $(call SetupHostCommand,perl,Please install Perl 5.x, \
 	perl --version | grep "perl.*v5"))
 

include/subdir.mk

@@ -43,6 +43,7 @@ log_make = \
 	 $(if $(BUILD_LOG), \
 		set -o pipefail; \
 		mkdir -p $(BUILD_LOG_DIR)/$(1)$(if $(4),/$(4));) \
+	gtime -f "time: $(1)$(if $(4),/$(4))/$(if $(3),$(3)-)$(2)\#%U\#%S\#%e" -- \
 	$$(SUBMAKE) $(subdir_make_opts) $(if $(3),$(3)-)$(2) \
 		$(if $(BUILD_LOG),SILENT= 2>&1 | tee $(BUILD_LOG_DIR)/$(1)$(if $(4),/$(4))/$(if $(3),$(3)-)$(2).txt)