fsverity_init: cleanly support kernels without builtin sig support

Since Android no longer uses fsverity builtin signatures, it's planned
to start configuring the kernel without
CONFIG_FS_VERITY_BUILTIN_SIGNATURES.  Therefore, make fsverity_init
cleanly handle the case of CONFIG_FS_VERITY_BUILTIN_SIGNATURES being
disabled.  Also document why fsverity_init still has to exist at all.

Bug: 290064770
Test: Booted Cuttlefish with android-mainline kernel with
      CONFIG_FS_VERITY_BUILTIN_SIGNATURES disabled.  Checked logcat for
      message indicating that 'fsverity_init --load-verified-keys'
      exited with status 0.
Change-Id: I0e232c9f4fb80f790ccafb03c10bb5dd5f24fe24
1 file changed