boards: st: nucleo_f207zg: Rework flash partition

STM32F2 series flash layout is as follows:
	{.pages_count = 4, .pages_size = KB(16)},
	{.pages_count = 1, .pages_size = KB(64)},
	{.pages_count = 7, .pages_size = KB(128)}

Since NVS subsys requires 2 sectors of max 32K in total, provide a
flash partition which respects this constraint using 2 of the 16K sectors
in the beginning of the layout.

Provide a compatible flash partition usable with mcuboot, but keep the
storage partition commented as its usage is not compatible with use w/o
mcuboot enabled (in this case main image starts as offset 0 which conflicts
with storage partition).

Note that it isn't possible either to get main image starting directly
in the 128K sectors w/o bootloader as boot flash address can't be
configured.


Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>