brcm2708: boot-part feature integration

brcm2708: boot-part feature integration

This patch adds the boot-part feature which enables the brcm2708
target move from the custom boot partition size config option to
the generic CONFIG_TARGET_KERNEL_PARTSIZE.

Note:
For people using custom images: Just like with
CONFIG_TARGET_ROOTFS_PARTSIZE changing the value
can cause sysupgrade to repartition the device!
Make sure to have a backup in this case.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>