Federal bank regulators set a severe timeline for Coral Gables-based BankUnited to get out of the crosshairs. It has 20 days to merge or find a buyer to raise its depleted capital to acceptable levels. The Office of Thrift Supervision order puts the ailing institution at imminent risk of a government takeover. But federal regulators haven’t sketched out the fate of BankUnited, which has been plagued by bad mortgage loans, if it doesn’t beat the clock. CEO Ramiro Ortiz said he’s continuing to try to raise capital and has had previous regulatory troubles.