boot: Revert "boot: Check shared area more carefully before init"

The commit "boot: Check shared area more carefully before init" adds
the support that the shared area can be used by other boot stages.
Before this commit, MCUboot initialize the shared memory based on a
global variable which indicates whether the shared memory has been
initialized by MCUboot. After this commit, MCUboot also checks the magic
value as well as the sanity check before initialization. So if the data
in shared memory retains after a reset, MCUboot does not initialize the
shared memory thus 'SHARED_MEMORY_OVERWRITE' error happens in
'boot_add_data_to_shared_area'.

So reverted this commit temporarily.

Signed-off-by: Sherry Zhang <sherry.zhang2@arm.com>
Change-Id: I1fc390bc17f90c2624024bc101ba8b4d5a75fe23