bzip2/lzma: don't ask for compression mode for the default initramfs

Impact: Kconfig noise reduction, documentation

The default initramfs is so small that it makes no sense to worry
about the additional memory taken by not double-compressing it.
Therefore, don't bug the user with it.

Also, improve the description of the option, which was downright
incorrect.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>