Clean-up boot command to use the predefined device and part for FAT
environment.
According to this device and partition, select the proper boot media.
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Heap base address is computed based on SYS_INIT_SP_ADDR by
subtracting the SYS_MALLOC_F_LEN value in
board_init_f_init_reserve().
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>