Direct commit: not relevant to other branches.

Fix mountroot> prompt eating most of the characters by not enabling
RXRDY interrupts in the attach routine. Instead, defer this until the
first interrupt we see after the device is opened. Given the console
use case, we're guaranteed to get a TXRDY interrupt before any reads
are posted due to boot messages, which makes this work.

The real fix is to use cngrab/cnungrab function pointers to disable
RXRDY interrupts while grabbed. However, that touches the MI uart
code, so was disallowed for 10.0 due to the lateness of the hour this
fix was proposed. It works for mountroot, the most common atmel kernel
prompt use cases, but wouldn't work for GELI since it prompts later in
the boot process.

Approved by: re@ (gjb@)

Direct commit: not relevant to other branches.

Fix mountroot> prompt eating most of the characters by not enabling
RXRDY interrupts in the attach routine. Instead, defer this until the
first interrupt we see after the device is opened. Given the console
use case, we're guaranteed to get a TXRDY interrupt before any reads
are posted due to boot messages, which makes this work.

The real fix is to use cngrab/cnungrab function pointers to disable
RXRDY interrupts while grabbed. However, that touches the MI uart
code, so was disallowed for 10.0 due to the lateness of the hour this
fix was proposed. It works for mountroot, the most common atmel kernel
prompt use cases, but wouldn't work for GELI since it prompts later in
the boot process.

Approved by: re@ (gjb@)