mbedtls: refactor mbedtls build for XPL

mbedtls: refactor mbedtls build for XPL

Refactor the entire kconfig page for mbedtls, adapt mbedtls makefile
and default config file using 'XPL_', in order to have independent
mbedtls kconfig options in U-Boot Proper, SPL, TPL and VPL.
User can choose legacy or mbedtls libraries for them independently.

Set mbedtls native hashing libraries as default when MBEDTLS_LIB,
SPL_MBEDTLS_LIB, TPL_MBEDTLS_LIB or VPL_MBEDTLS_LIB is selected.

If users prefer using U-Boot legacy hashing libraries, please select
MBEDTLS_LIB_HASHING_ALT, SPL_MBEDTLS_LIB_HASHING_ALT,
TPL_MBEDTLS_LIB_HASHING_ALT or VPL_MBEDTLS_LIB_HASHING_ALT for U-Boot
Proper, SPL, TPL and VPL respectively.

Moreover, rename a few kconfig options and update their descriptions to
improve the consistency of terminology.

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