Added sama5 to BUILD_SUBTARGET variable.This will populate at91bootstrap
menu options in bootloader menu only when SAMA5 devices are selected as
SUBTARGET and to avoid showing up this menu when legacy device is
selected as SUBTARGET and fixed typo mistake: sama5d3 -> sama5d2.
Signed-off-by: Sandeep Sheriker Mallikarjun <sandeepsheriker.mallikarjun@microchip.com>