* Signed boot and HTTP boot mode
HTTP boot mode is supposed to be disabled if signed boot is enabled and
a host is not specified. The code is checking the http_secure flag to
enforce this. But this is valid now we support custom CA certs.
Only disable HTTP mode if we're using the default HOST.
* Implement TCP window for net boot
The minimal IP stack used for https booting lacks the ability to cache
packets received out of order, which can lead to severe slowdown when
it happens. The problem seems to affect some ISPs more than others.
The receive window implemented here copes with packet losses of 10%.
* netboot: Correct the TCP MSS
* Correct msecs in debug timestamps
The fractional part of timestamps in UART debug output was showing the
100ths and 1000ths of a second, rather than 10ths and 100ths, causing
strange sequences that appear to jump backwards.
