RX-51: Add support for bootmenu

 * default bootmenu entries:
   attached kernel, internal eMMC memory, external SD card,
   u-boot boot order

 * in CONFIG_PREBOOT try load bootmenu.scr from first partition
   of internal eMMC memory (also known as MyDocs) which (should)
   overwrite default bootmenu entries

 * when keyboard slide is closed boot first menu entry

 * when keyborad slide is open show bootmenu

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>