support TARGET_EXTRA_RECOVERY_KEYS

Specifying one or more key files (without .x509.pem extension) as
TARGET_EXTRA_RECOVERY_KEYS causes them to be included as acceptable
keys for recovery packages.  They are *not* included in otacerts.zip,
so actual downloaded over-the-air packages can't use them, but they
can be used to sign sideload-only packages.

Bug: 3413359
Change-Id: I6f248ffa35f0c6b125dd8a7517493017e236c776