installboot: powerpc64: fix passing explicit stage files

[Klemens Nanni]

This fixes installboot regress on powerpc64, forgot to send it earlier.

The same diff already landed for macppc;  efi already the same fix
md_init() but without the cleanup that entails for macppc and powerpc64.

>From macppc_installboot.c r1.6 "Fix passing explicit stage files":

        Using `stage1' leads to a bit more cleanup since early MI installboot.c
        handles `-r', i.e. write_filesystem() no longer has needs to do the
        fileprefix() dance itself.

OK?

Index: powerpc64_installboot.c
===================================================================
RCS file: /cvs/src/usr.sbin/installboot/powerpc64_installboot.c,v
retrieving revision 1.5
diff -u -p -r1.5 powerpc64_installboot.c
--- powerpc64_installboot.c     9 Sep 2022 15:53:16 -0000       1.5
+++ powerpc64_installboot.c     12 Sep 2022 12:31:44 -0000
@@ -63,6 +63,8 @@ char  duid[20];
 void
 md_init(void)
 {
+       stages = 1;
+       stage1 = "/usr/mdec/boot";
 }
 
 void
@@ -172,12 +174,9 @@ write_filesystem(struct disklabel *dl, c
        char cmd[60];
        char dir[PATH_MAX];
        char dst[PATH_MAX];
-       char *src;
-       size_t mntlen, srclen;
+       size_t mntlen;
        int rslt;
 
-       src = NULL;
-
        /* Create directory for temporary mount point. */
        strlcpy(dir, "/tmp/installboot.XXXXXXXXXX", sizeof(dst));
        if (mkdtemp(dir) == NULL)
@@ -239,17 +238,11 @@ write_filesystem(struct disklabel *dl, c
                warn("unable to build /boot path");
                goto umount;
        }
-       src = fileprefix(root, "/usr/mdec/boot");
-       if (src == NULL) {
-               rslt = -1;
-               goto umount;
-       }
-       srclen = strlen(src);
        if (verbose)
                fprintf(stderr, "%s %s to %s\n",
-                   (nowrite ? "would copy" : "copying"), src, dst);
+                   (nowrite ? "would copy" : "copying"), stage1, dst);
        if (!nowrite) {
-               rslt = filecopy(src, dst);
+               rslt = filecopy(stage1, dst);
                if (rslt == -1)
                        goto umount;
        }
@@ -292,8 +285,6 @@ rmdir:
        dst[mntlen] = '\0';
        if (rmdir(dir) == -1)
                err(1, "rmdir('%s') failed", dir);
-
-       free(src);
 
        if (rslt == -1)
                exit(1);