efi_loader: Fix serial console size detection

efi_loader: Fix serial console size detection

Function term_read_reply tries to read from the serial console until
the end_char was read. This can hang forever if we are, for some reason,
not able to read the full response (e.g. serial buffer too small,
frame error). This patch moves the timeout detection into
term_read_reply() to assure we will make progress.

Fixes: 6bb591f704 ("efi_loader: query serial console size reliably")
Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Throw missing error when an incomplete reply for the cursor position is
received.

Change type of argument of term_get_char() *s32. This renders the function
reusable in efi_cin_read_key().

Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>