build: add xargs as prerequisite

Build system needs an 'xargs' that supports '-r' which darwin doesn't.
Homebrew installs a 'gxargs' with the findutils package so look for
'gxargs' as well as 'xargs'

This is a bit of a 'fun' corner case anyway. xargs is only required by
the build if 'CONFIG_AUTOREMOVE' is set and after the build system has
built 'tools/findutils' we have a fully working xargs for host anyway.
Until that time we have to rely on the host's xargs implementation.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

include/prereq-build.mk

@@ -93,6 +93,11 @@ $(eval $(call SetupHostCommand,find,Please install GNU 'find', \
 $(eval $(call SetupHostCommand,bash,Please install GNU 'bash', \
 	bash --version 2>&1 | grep GNU))
 
+$(eval $(call SetupHostCommand,xargs, \
+	Please install 'xargs' that supports '-r/--no-run-if-empty', \
+	gxargs -r --version, \
+	xargs -r --version))
+
 $(eval $(call SetupHostCommand,patch,Please install GNU 'patch', \
 	gpatch --version 2>&1 | grep 'Free Software Foundation', \
 	patch --version 2>&1 | grep 'Free Software Foundation'))