Make crossystem.h more polite and more useful.

This adds a VB_MAX_STRING_PROPERTY for callers that don't
want to guess at how big to make their buffers.

Additionally, it changes the size parameter to VbGetPropertyString()
from int to size_t.

BUG=None
TEST=compile the code
BRANCH=none

Change-Id: I22809d48e13b535593cb22a56444e2dcb27791a5
Reviewed-on: https://chromium-review.googlesource.com/175039
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Tested-by: Richard Barnette <jrbarnette@chromium.org>
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
Commit-Queue: Richard Barnette <jrbarnette@chromium.org>

host/include/crossystem.h

@@ -10,6 +10,12 @@
 extern "C" {
 #endif
 
+#include <stddef.h>
+
+/* Recommended size for string property buffers used with
+ * VbGetSystemPropertyString(). */
+#define VB_MAX_STRING_PROPERTY     ((size_t) 8192)
+
 /* Reads a system property integer.
  *
  * Returns the property value, or -1 if error. */
@@ -19,8 +25,12 @@ int VbGetSystemPropertyInt(const char* name);
  * specified size.  Returned string will be null-terminated.  If the
  * buffer is too small, the returned string will be truncated.
  *
+ * The caller can expect an un-truncated value if the size provided is
+ * at least VB_MAX_STRING_PROPERTY.
+ *
  * Returns the passed buffer, or NULL if error. */
-const char* VbGetSystemPropertyString(const char* name, char* dest, int size);
+const char* VbGetSystemPropertyString(const char* name, char* dest,
+                                      size_t size);
 
 /* Sets a system property integer.
  *
@@ -28,6 +38,9 @@ const char* VbGetSystemPropertyString(const char* name, char* dest, int size);
 int VbSetSystemPropertyInt(const char* name, int value);
 
 /* Set a system property string.
+ *
+ * The maximum length of the value accepted depends on the specific
+ * property, not on VB_MAX_STRING_PROPERTY.
  *
  * Returns 0 if success, -1 if error. */
 int VbSetSystemPropertyString(const char* name, const char* value);