recovery: Add timestamps in update logs.

Fork a logger process and send over the log lines through a pipe.
Prepend a timestamp to each line for debugging purpose. Timestamps are
relative to the start of the logger.

Example lines with the change in this CL:

[  445.948393] Verifying update package...
[  446.279139] I:comment is 1738 bytes; signature 1720 bytes from end
[  449.463652] I:whole-file signature verified against RSA key 0
[  449.463704] I:verify_file returned 0

Change-Id: I139d02ed8f2e944c1618c91d5cc43282efd50b99