[hci] Continue processing while prompting for shell banner

[hci] Continue processing while prompting for shell banner

Currently we do nothing while polling for user input in shell_banner.
This commit modifies shell_bannder to do something similar to what is
done in shell, where we schedule processes while waiting for user
input.

This approach provides two potential improvements:

First:
	We increase key responsiveness in shell_banner. It is much more
	likely to successfully get into shell than before. Before this
	change I sometimes pressed the CTRL_B, but failed to get into
	shell. It may have been caused by waiting too long in
	mdelay(100).

Second:
	When using scripts, if the downloaded image is not bootable,
	gPXE exits before it has the opportunity to gracefully close
	the TCP connection. This can leave the TCP state of a remote
	server as not closed when it should have been.

	gPXE waits for user to input for 2 seconds by default before
	returning control to BIOS. This commit lets TCP have the
	opportunity to gracefully close while waiting for the user to
	hit keys in shell_banner.

Referenced: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Guo-Fu Tseng <cooldavid@cooldavid.org>
Commit-message-modified-by: Marty Connor <mdc@etherboot.org>
Signed-off-by: Marty Connor <mdc@etherboot.org>