Do not put GENERIC_KERNEL_CMDLINE in recovery image.

The GENERIC_KERNEL_CMDLINE should only be in the generic boot image.
If device uses recovery-as-boot, it never uses generic boot image
because on devices with generic boot image, recovery resources are
moved to vendor_boot instead.

Bug: 171512004
Test: builds

Change-Id: Ia84e604a8ded28af39c7f1861ff5d3b3af55849f