Revert "loader: support.4th resets the read buffer incorrectly"

This reverts commit 9c1c02093b90ae49745a174eb26ea85dd1990eec. It seems
to have broken all old nextboot.conf files causing hangs on boot.

PR: 239315
Sponsored by: Netflix

(cherry picked from commit 4783fb730fa1cfdbe5c905bb23ac74f681e2df6b)