Too big kernel for u-boot

[KaneTW]

MACHINE=at91sam9x5ek-sd bitbake microchip-graphics-image on kirkstone leads to a kernel that is over 8MB, leading to "uncompressed: uncompress error -28". This is fixed by setting CONFIG_SYS_BOOTM_LEN to a higher value, e.g. 16MB. I hacked it for myself by changing the default in boot/bootm.c, but there's surely a better way.