BOOT_IMAGE isn't set by petitboot

[sharkcz]

Looks like petitboot isn't adding the BOOT_IMAGE variable to the kernel command line, while other bootloaders do that (grub2 on ppc/pseries, zipl on s390x - confirmed by myself, but grub does it for x86 and aarch64 too).

dan@fedora30-ppc ~]$ cat /proc/cmdline 
BOOT_IMAGE=/vmlinuz-5.3.15-200.fc30.ppc64le root=/dev/mapper/fedora-root ro rd.lvm.lv=fedora/root rd.lvm.lv=fedora/swap rhgb quiet
[dan@fedora30-ppc ~]$ rpm -q grub2-ppc64le
grub2-ppc64le-2.02-84.fc30.ppc64le
[dan@fedora30-ppc ~]$ cat /proc/cpuinfo 
processor	: 0
cpu		: POWER9 (architected), altivec supported
clock		: 3000.000000MHz
revision	: 2.2 (pvr 004e 1202)

timebase	: 512000000
platform	: pSeries
model		: IBM pSeries (emulated by qemu)
machine		: CHRP IBM pSeries (emulated by qemu)
MMU		: Radix

CC @martinezjavier @jhlavac