board-qemu: Fix slof-build-id length

The existing code hardcodes the length of /openprom/model to 10 characters
even though it is less than that - len("aik")==3. All 10 chars go to
the device tree blob and DTC complains on such a property as there are
characters after terminating null:

aik@fstn1-p1:~$ dtc -f -I dtb -O dts -o dbg.dts dbg.dtb
Warning (model_is_string): "model" property in /openprom is not a string

This uses the real length and limits it by 10 to avoid breaking something.

Since the same code parses the build id field, this moves from-cstring
to a common place for both js2x and qemu boards.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>

board-js2x/slof/helper.fs

@@ -11,7 +11,7 @@
 \ ****************************************************************************/
 
 : slof-build-id  ( -- str len )
-   flash-header 10 + a
+   flash-header 10 + dup from-cstring a min
 ;
 
 : slof-revision s" 001" ;

board-qemu/slof/helper.fs

@@ -11,7 +11,7 @@
 \ ****************************************************************************/
 
 : slof-build-id  ( -- str len )
-   flash-header 10 + a
+   flash-header 10 + dup from-cstring a min
 ;
 
 : slof-revision s" 001" ;
@@ -27,12 +27,6 @@
    bdate2human $cat encode-string THEN
 ;
 
-\ Fetch C string
-: from-cstring ( addr - len )  
-  dup dup BEGIN c@ 0 <> WHILE 1 + dup REPEAT
-  swap -
-;
-
 : invert-region-cs ( addr len cellsize -- )
    >r over swap r@ rshift r> swap 1 hv-logical-memop drop
 ;

slof/fs/base.fs

@@ -107,6 +107,11 @@ CONSTANT <2constant>
 : str= ( str1 len1 str2 len2 -- equal? )
   rot over <> IF 3drop false ELSE comp 0= THEN ;
 
+: from-cstring ( addr - len )
+  dup dup BEGIN c@ 0 <> WHILE 1 + dup REPEAT
+  swap -
+;
+
 : test-string ( param len -- true | false )
    0 ?DO
       dup i + c@                     \ Get character / byte at current index