pseries: Update SLOF firmware image to qemu-slof-20180621

The changes are:
1. fixed broken_sc1;
2. added switching between boot consoles;
3. added PXE boot.

The full list is:
 > lib/libnet/pxelinux: Fix two off-by-one bugs in the pxelinux.cfg parser
 > lib/libnet/pxelinux: Make the size handling for pxelinux_load_cfg more logical
 > libc: Add a simple implementation of an assert() function
 > libnet: Support UUID-based pxelinux.cfg file names
 > slof: Add a helper function to get the contents of a property in C code
 > libnet: Add support for DHCPv4 options 209 and 210
 > libnet: Wire up pxelinux.cfg network booting
 > libnet: Add functions for downloading and parsing pxelinux.cfg files
 > libnet: Put code for determing TFTP error strings into a separate function
 > libc: Add the snprintf() function
 > libnet: Pass ip_version via struct filename_ip
 > resolve ihandle and xt handle in the input command (like for the output)
 > Fix output word
 > obp-tftp: Make sure to not overwrite paflof in memory
 > libnet: Get rid of unused huge_load and block_size parameters
 > libc: Check for NULL pointers in free()
 > libc: Implement strrchr()
 > libnet: Get rid of unnecessary (char *) casts
 > broken_sc1: check for H_PRIVILEGE
 > OF: Use new property "stdout-path" for boot console

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>