zephyr: fix Mbed TLS configuration header file selection

Mbed TLS already provides MBEDTLS_CFG_FILE Kconfig when it's builtin. So
we only need to override that config when not using the builtin Mbed TLS
version.

This commit also fixes some hardcoding of "mcuboot-mbedtls-cfg.h" in
Kconfig and prj.conf.

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