wic: Use mke2fs to create the ext2/3/4 image

OE-core commit f3a95ca6886b55e5819b068bdbd2cceb882d91a6 removed the
populate-extfs.sh. So mke2fs should be used to create ext2/3/4 image.

Signed-off-by: João Henrique Ferreira de Freitas <joaohf@gmail.com>
Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
wic: Use pseudo for ext mkfs command

The switchover from populate-exfs.sh to mke2fs forgot to preserve
pseudo, add it back.

Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>