Remove INITRAMFS_TARGET variable

This used to be needed when the filesystem code was rewritten, but not
the Linux compilation code. Now that the Linux compilation code has
been rewritten, the mechanism to ensure that initramfs gets built
*before* the kernel so that it can be integrated is different, and
this INITRAMFS_TARGET variable is no longer used.

See f507921d for details.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>