lib: Adapt digest header files to MbedTLS

lib: Adapt digest header files to MbedTLS

Adapt digest header files to support both original libs and MbedTLS
by switching on/off MBEDTLS_LIB_CRYPTO.
Introduce <alg>_LEGACY kconfig for legacy hash implementations.
sha256.o should depend on SHA256 kconfig only but not SUPPORT_EMMC_RPMB,
SHA256 should be selected when SUPPORT_EMMC_RPMB is enabled instead.

`IS_ENABLED` or `CONFIG_IS_ENABLED` is not applicable here, since
including <linux/kconfig.h> causes undefined reference on schedule()
with sandbox build, as <linux/kconfig.h> includes <generated/autoconf.h>
which enables `CONFIG_HW_WATCHDOG` and `CONFIG_WATCHDOG` but no schedule()
are defined in sandbox build,
Thus we use `#if defined(CONFIG_MBEDTLS_LIB_CRYPTO)` instead.

Signed-off-by: Raymond Mao <raymond.mao@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>