This patch adds the boot-part feature to the apm82181 sata target.
This makes it possible to configure the boot partition size with
the generic CONFIG_TARGET_KERNEL_PARTSIZE symbol.
Please note: For people using custom images: Just like with
CONFIG_TARGET_ROOTFS_PARTSIZE changing the value can cause
sysupgrade to repartition the device!
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>