xbmk: use sbase sha512sum, not host sha512sum

the --status flag seems to be a GNUism

as stated in the previous commit, i import sbase
suckless now, so as to have a consistent implementation
of sha512sum.

this ensures that its output is reliable, when i'm using
the output of this command within backticks.

Signed-off-by: Leah Rowe <leah@libreboot.org>

include/lib.sh

@@ -162,6 +162,13 @@ unpad_one_byte()
 	x_ mv "$unpaddedfile" "$1"
 }
 
+build_sbase()
+{
+	if [ ! -f "$sha512sum" ]; then
+		x_ make -C "$xbmkpwd/util/sbase"
+	fi
+}
+
 fx_()
 {
 	xchk fx_ "$@"