Add option that allows you to explicitly disable use of the tpm
otherwise if a tss stack such as trousers is found by gnutls build
system then it will use it which will cause a build failure when
PKCS11 support is not enabled.

Signed-off-by: Ian Pozella <Ian.Pozella@imgtec.com>

It is not enough to select package kmod-cryptodev to avoid
build failure; there must be a Makefile dependency on
kmod-cryptodev in order for the build system to ensure
kmod-cryptodev (and it's headers) are present during build.

Therefore drop the select and add a conditional dependency.

Signed-off-by: Daniel Dickinson <openwrt@daniel.thecshore.com>

Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>

Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>

Signed-off-by: Nikos Mavrogiannopoulos <nmav@gnutls.org>