efi_loader: check tcg2 protocol installation outside the TCG protocol

There are functions that calls tcg2_agile_log_append() outside
of the TCG protocol invocation (e.g tcg2_measure_pe_image).
These functions must to check that TCG2 protocol is installed.
If not, measurement shall be skipped.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>