tpm: Make SECURITYFS a weak dependency

While having SECURITYFS enabled for the tpm subsystem is beneficial in
most cases, it is not strictly necessary to have it enabled at all.
Especially on platforms without any boot firmware integration of the TPM
(e.g. raspberry pi) it does not add any value for the tpm subsystem,
as there is no eventlog present.

By turning it from 'select' to 'imply' it still gets selected per
default, but enables users who want to save some kb of ram by turning
SECURITYFS off.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>