e2fsprogs: remove busybox tune2fs when installing full version

Currently, busybox installs a version of tune2fs in /sbin/ while
e2fsprogs installs at /usr/sbin/. This causes issues if a user is
expecting the to use the full version from a prompt via 'tune2fs'
since busybox's tune2fs will be found first from the default path
search.

Fix this by removing the busybox version of tune2fs when a user has
selected the option to install tune2fs from e2fsprogs.

[Thomas: don't hijack E2FSPROGS_TARGET_TUNE2FS_SYMLINK for the removal
and instead introduce a separate E2FSPROGS_REMOVE_BUSYBOX_APPLETS post
install target hook, and do related code re-organization.]

Signed-off-by: Ryan Barnett <ryan.barnett@rockwellcollins.com>
Reviewed-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>