contrib/rootfs-builder: Use $(cat rootfs-files)

The previous

  $(< rootfs-files)

is a Bashism that's not part of POSIX command substitution [1].  For
Bash, the $(< ...) form is slightly faster (no need to exec cat), but
not everyone uses Bash.

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_03

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

contrib/rootfs-builder/Makefile

@@ -15,7 +15,7 @@ rootfs/%/bin/busybox: downloads/stage3-%-current.tar rootfs-files
 	sudo rm -rf rootfs/$*
 	sudo mkdir -p rootfs/$*
 	sudo tar -xvf downloads/stage3-$*-current.tar -C rootfs/$* \
-		--no-recursion --wildcards $$(< rootfs-files)
+		--no-recursion --wildcards $$(cat rootfs-files)
 	sudo touch $@
 
 .PRECIOUS: rootfs/%/bin/echo