bootm: Add board specific OS preboot hook

Add board-specific hook which is executed before the code hands over
control to the OS. This lets the board perform some last-minute clean
ups.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>

common/bootm_os.c

@@ -505,10 +505,17 @@ __weak void arch_preboot_os(void)
 	/* please define platform specific arch_preboot_os() */
 }
 
+/* Allow for board specific config before we boot */
+__weak void board_preboot_os(void)
+{
+	/* please define board specific board_preboot_os() */
+}
+
 int boot_selected_os(int argc, char * const argv[], int state,
 		     bootm_headers_t *images, boot_os_fn *boot_fn)
 {
 	arch_preboot_os();
+	board_preboot_os();
 	boot_fn(state, argc, argv, images);
 
 	/* Stand-alone may return when 'autostart' is 'no' */