build: add busybox support to `time` prereq-check

Busybox time supports the GNU time '-f' syntax used by the build time
logging implemented in ff6e62b288, however the prerequisite check added
only works with GNU time installed as `time` or `gtime`.

As busybox is a multicall binary, the name of the symlink setup by
SetupHostCommand also must be changed from `gtime` to `time` to fix the
value of argv[0]. This causes a number of shells (including bash) to use
their builtin impelementation of time, so the sole invocation has been
changed to use `env time` to use the value found on the $PATH.

Signed-off-by: Andy Boyett <agb@agb.io>

include/prereq-build.mk

@@ -138,9 +138,10 @@ $(eval $(call SetupHostCommand,bzip2,Please install 'bzip2', \
 $(eval $(call SetupHostCommand,wget,Please install GNU 'wget', \
 	wget --version | grep GNU))
 
-$(eval $(call SetupHostCommand,gtime,Please install GNU 'time', \
+$(eval $(call SetupHostCommand,time,Please install GNU 'time' or BusyBox 'time', \
 	gtime --version 2>&1 | grep GNU, \
-	time --version 2>&1 | grep GNU))
+	time --version 2>&1 | grep GNU, \
+	busybox time 2>&1 | grep BusyBox))
 
 $(eval $(call SetupHostCommand,perl,Please install Perl 5.x, \
 	perl --version | grep "perl.*v5"))

include/subdir.mk

@@ -43,7 +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" -- \
+	env time -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)