samples: mcumgr: smp_svr: disable "zephyr,entropy" in mcuboot

If/when "zephry,entropy" is enabled CONFIG_ENTROPY_GENRATOR is
automatically enabled, therefore enabling the corresponding
platform entropy driver. On some platform this is a problem
because the entropy driver is not used directly as random
source, but only to seed the ctr-drbg algorithm provided
by Mbed TLS. Unfortunately CONFIG_MBEDTLS_CIPHER_MODE_CTR_ENABLED
cannot be directly enabled because CONFIG_MBEDTLS_CFG_FILE used
in mcuboot is different from the "config-tls-generic.h" which
is required to use Kconfig symbols defined in
"zephyr/modules/mbedtls/Kconfig.tls-generic".
Moreover entropy and random number generators are not required
in mcuboot because signature verification is a
deterministic operation.
In conclusion this commit disables "zephyr,entropy" in
mcuboot DTS overlay file.

Signed-off-by: Valerio Setti <vsetti@baylibre.com>