unit-test: make "test -e" test independent of $CWD

The unit-test for hush's "test -e" currently relies upon being run in
the U-Boot build directory, because it tests for the existence of a file
that exists in that directory.

Fix this by explicitly creating the file we use for the existence test,
and deleting it afterwards so that multiple successive unit-test
invocations succeed. This required adding an os.c function to erase
files.

Reported-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>

include/os.h

@@ -83,6 +83,14 @@ int os_open(const char *pathname, int flags);
  */
 int os_close(int fd);
 
+/**
+ * Access to the OS unlink() system call
+ *
+ * \param pathname Path of file to delete
+ * \return 0 for success, other for error
+ */
+int os_unlink(const char *pathname);
+
 /**
  * Access to the OS exit() system call
  *