config/git: import suckless sbase

i currently use the output of sha512sum in several
places of xbmk, which is a bit unreliable in case
output changes.

other cases where i use util outputs in variables
are probably reliable, because i'm using mostly
posix utilities in those.

to mitigate this, i now import suckless sbase, which
has a reasonable sha512sum implementation.

*every* binary it builds is being placed in build.list,
because i'll probably start using more of them.

for example, i may start modifying the "date"
implementation, adding the GNU-specific options that
i need as mentioned on init.sh

i'm importing it in util/ because the sha512sum
util is needed for verifying project sources, so
if sbase itself is a "project source", that means
we can into a chicken and egg bootstrapping problem.

this is sbase at revision:
055cc1ae1b3a13c3d8f25af0a4a3316590efcd48

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

util/sbase/readlink.c

@@ -0,0 +1,54 @@
+/* See LICENSE file for copyright and license details. */
+#include <limits.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#include "util.h"
+
+static void
+usage(void)
+{
+	eprintf("usage: %s [-fn] path\n", argv0);
+}
+
+int
+main(int argc, char *argv[])
+{
+	char buf[PATH_MAX];
+	ssize_t n;
+	int nflag = 0, fflag = 0;
+
+	ARGBEGIN {
+	case 'f':
+		fflag = ARGC();
+		break;
+	case 'n':
+		nflag = 1;
+		break;
+	default:
+		usage();
+	} ARGEND
+
+	if (argc != 1)
+		usage();
+
+	if (strlen(argv[0]) >= PATH_MAX)
+		eprintf("path too long\n");
+
+	if (fflag) {
+		if (!realpath(argv[0], buf))
+			eprintf("realpath %s:", argv[0]);
+	} else {
+		if ((n = readlink(argv[0], buf, PATH_MAX - 1)) < 0)
+			eprintf("readlink %s:", argv[0]);
+		buf[n] = '\0';
+	}
+
+	fputs(buf, stdout);
+	if (!nflag)
+		putchar('\n');
+
+	return fshut(stdout, "<stdout>");
+}