libc: common: Fix init hang issue

In device init phase, it will call _mbedtls_init before malloc_prepare
as mbedtls has higher priority defined in SYS_INIT..
_mbedtls_init() will call psa_crypto_init() and malloc buffer,
but z_malloc_heap is not initialized, which will cause device hang.
Should call malloc_prepare() before _mbedtls_init to fix this issue,
so add new Kconfig to increase the priority of libc to deafult 30.

Signed-off-by: Maochen Wang <maochen.wang@nxp.com>