boot: Handle shift and logo keys too

Some firmware supports sending input events for shift and logo keys.
Previously, we would suppress these with EFI_NOT_READY unless
some other key was pressed alongside, but it is really the job of the
caller to decide how to handle these.

Note that for keys that already have a printable shift representation
the reported input event will not have the shift key bits set
(Shift+a is reported as A). Should some firmware turn out to violate the
spec here we can always remove that part.