phase1: opportunistic use of ccache

phase1: opportunistic use of ccache

Assuming that CONFIG_CCACHE is disabled in the target config.seed,
this patch tries to enable the use of ccache (if available from the host)
for the 'dltar' and 'tools' steps, which are the only two steps where the
build objects are shared by /all/ workers, and thus the two steps where
caching makes the most sense and should provide the most benefits.

Signed-off-by: Thibaut VARÈNE <[email protected]>