init: add support for s390x installation on LPAR

[tuan-hoang1]

In QEMU/KVM, ttysclp0 (ttyS1) is automatically detected and used by
default.
In z/VM, ttyS0 is still used instead of ttysclp0/sclp_line0, however.
In LPAR, ttyS0 is mapped to sclp_line0 as expected.

This patch detects if we are running in LPAR mode and change ttyS0 to
sclp_line0 accordingly. /proc/sysinfo is s390x specific.

[tuan-hoang1]

Not sure if I should include URL in the git commit, but I paste here for references:
https://www.ibm.com/support/knowledgecenter/en/linuxonibm/com.ibm.linux.z.luvg/luvg_r_lmtfeatures.html
https://www.ibm.com/support/knowledgecenter/en/linuxonibm/com.ibm.linux.z.lgdd/lgdd_r_console_sum.html
https://www.ibm.com/support/knowledgecenter/en/linuxonibm/com.ibm.linux.z.lhdd/lhdd_r_console_assure_nodes.html

[tuan-hoang1]

I understand introducing more arch-specific stuffs are not very friendly but I don't have much options. Any comment is welcomed !

[tuan-hoang1]

This seems to be a wrong way to do it. Working on it.

[tuan-hoang1]

This commit is ready. cc @ncopa

[tuan-hoang1]

Still small aesthetic hiccup. Will update again in next commit in next few hours.

[tuan-hoang1]

Run and tested perfectly on KVM, z/VM and LPAR installations. Ready to go.

[tuan-hoang1]

To explain the logic:

[ "$1" = "ttyS0" ] && [ -e /sys/class/tty/sclp_line0 ] && return

I have observed that ttyS0 is available in /sys/class/tty/console/active even though /sys/class/tty/ttyS0 directory is not present on LPAR installation. On LPAR installation, sclp_line0 (the device name for ttyS0 console) is available instead, thus this if block. Putting console=ttyS0 or console=sclp_line0 won't work. Without this block, stdout/stderr is spammed with messages of ttyS0 not found.

	for _tty in hvc0 xvc0 hvsi0 sclp_line0 ttysclp0 "3270!tty1"; do
		[ -e /sys/class/tty/$_tty ] && echo $_tty
	done

On s390x, if a tty is available, the directory /sys/class/tty/$tty_name exists, which is enough.

These 2 blocks wouldn't affect any other architectures I believe.