96b_nitrogen: Increase size of boot partition.

MCUboot needs nearly all 8 pages, on some commits even more. This leads
to a overwrite of parts of mcuboot code when an application is flashed.
By improving the size of the boot partition to 10 pages (from 8) and
decrease both application slots by 1 page, MCUboot fits in the boot
loader partition. This is also fixable by an extra .dts, but having an
out of the box working configuration is preferable.

Signed-off-by: Stefan Kraus <stefan.kraus@fau.de>