tpm: Rename function grub_tpm_log_event() to grub_tpm_measure()
    
    grub_tpm_log_event() and grub_tpm_measure() are two functions that
    have the same effect. So, keep grub_tpm_log_event() and rename it
    to grub_tpm_measure(). This way we get also a more clear semantics.
    
    Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
    Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>