wic: do not steal sectors from the first partition

wic: do not steal sectors from the first partition

In order to make up for space loss due to overhead located before the
first partition, same amount of bytes was extracted from the first
partition's size. This leads to unexpected change of first partition's
size, and is not matched by similar adjustment at the time the partition
image is generated. Thus it is possible that the first partition may
partially overwrite contents of the following partition, as the
partition image is larger than the allocated space.

This is also problematic on certain platforms such as OMAP, where the
first partition is required for have an even number of
sectors (typically already expressed by --size in kickstart
file). Subtracting an odd number of sectors lost for overhead, leads to
an odd number of sectors assigned for partition, preventing the board,
such as beaglebone, from booting correctly.

(From OE-Core rev: c3251d4191fb0b7c3477a3579e542729598f6aad)

Signed-off-by: Maciej Borzecki <maciej.borzecki@open-rnd.pl>
Signed-off-by: Maciek Borzecki <maciek.borzecki@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>