rules.mk: do not set CCACHE_NOHASHDIR

rules.mk: do not set CCACHE_NOHASHDIR

Not hashing CWD is potentially unsafe since it involves deliberately
poisoning the cache in certain situations in exchange for performance
gain. It can lead to debug information pointing out either no longer
existing or much worse incorrect source files, possibly leading
developers onto a false track and wasting a lot of time.

If one wishes to save build time by sharing the cache between multiple
source trees, this can be achieved safely by enabling reproducible
debug information, like this:

CONFIG_CCACHE_DIR="$(HOME)/.ccache"
CONFIG_REPRODUCIBLE_DEBUG_INFO=y

Note that CWD hashing gets disabled implicitly when reproducible debug
information is enabled. The CCACHE_NOHASHDIR option is only for
disabling CWD hashing in cases where it is not safe to do so.

Signed-off-by: Erik Karlsson <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20317
Signed-off-by: Robert Marko <[email protected]>