How is CONFIG_TPM selected?

[Trammell Hudson]

Is it possible to enable CONFIG_TPM with the current head in git?
On my Lenovo x230, CONFIG_MAINBOARD_HAS_LPC_TPM is selected, as is
CONFIG_LPC_TPM, but there does not appear to be any way to enable
CONFIG_TPM in menuconfig.  In order to enable it, I had to change
src/Kconfig to default to y.

Without it selected, src/lib/tclc.c won't compile since the prototype for
tlcl_define_space() in src/include/tpm_lite/tlcl.h depends on CONFIG_TPM
or CONFIG_TPM2 being defined.