busybox: disable CONFIG_FEATURE_CLEAN_UP in default configs

busybox: disable CONFIG_FEATURE_CLEAN_UP in default configs

FEATURE_CLEAN_UP is a configuration feature to get busybox to explicitly
call free() on dynamic allocated memory just before exiting so memory leak
detectors like valgrind don't get confused.  Upstream explicitly recommends
to NOT enable this option:

config FEATURE_CLEAN_UP
	bool "Clean up all memory before exiting (usually not needed)"
	default n
	help
	  As a size optimization, busybox normally exits without explicitly
	  freeing dynamically allocated memory or closing files. This saves
	  space since the OS will clean up for us, but it can confuse debuggers
	  like valgrind, which report tons of memory and resource leaks.

	  Don't enable this unless you have a really good reason to clean
	  things up manually.

Having this option enabled adds a bit of bloat, but more significantly these
cleanup code paths don't get tested very often so some times get out of sync
with the allocation code which can lead to crashes (or security issues from
double frees), so it is safer to disable the option.

For people wanting to debug memory leak issues with busybox, the option can
still be enabled with a configuration fragment (or a custom config).

The size difference isn't huge (br-arm-full-static):

-rwxr-xr-x 1 peko peko 886K Jul  5 10:56 output-busybox1/target/bin/busybox
-rwxr-xr-x 1 peko peko 882K Jul  5 10:53 output-busybox2/target/bin/busybox

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>