mbedtls: restore previous configuration behavior

mbedtls: restore previous configuration behavior

Fallback to default mbedtls configurations in case of the package is
not configured. It is possible for some reasons it get built even if
it's unselected because of build system bugs or other build-only
dependencies. In this case current behavior will comment out all
necessary configurations and lead build errors.

Fixes: 5359639c2b74 ("mbedtls: Apply configuration in Configure instead of Prepare")
Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>
Link: https://github.com/openwrt/openwrt/pull/19495
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>