tfm: Kconfig: Configure MCUBOOT_DATA_SHARING explicitly

Previously, MCUBOOT_DATA_SHARING was enabled when BL2 was built
and when the firmware update partition was present. This is not
the only situation that you might be interested in this data
sharing. The data sharing now has it's own Kconfig.

Further, use of the firmware update partition requires data
sharing, so it selects it.

Signed-off-by: Jimmy Brisson <jimmy.brisson@linaro.org>