images: remove redundant IMAGE_BASENAME assignments

images: remove redundant IMAGE_BASENAME assignments

core-image-tiny-initramfs and core-image-testcontroller-initramfs just
repeat PN, which is the default value.

core-image-minimal-initramfs adds MLPREFIX, but that's also covered by
PN:

$ bitbake-getvar -r lib32-core-image-minimal-initramfs IMAGE_BASENAME
IMAGE_BASENAME="lib32-core-image-minimal-initramfs"

As these assignments are all redundant, remove them.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>