linux-postmarketos-exynos5: use postmarketos-installkernel

Currently kernel image is installed to /boot/vmlinuz-postmarketos-exynos5, but it does not work because it is configured in u-boot script as /boot/vmlinuz. Drop localversion to get it working again.

For now I am not sure what broke it and how to fix that properly, but I would prefer to get this merged because in the current state it is unbootable

CC @Iourine @grimler @craftyguy