perl: clean prior to build

perl: clean prior to build

Otherwise, when running configure/make on a previously built tree,
makefiles generated by makemaker may detect that 'configuration has changed',
followed by self-deletion, self-regeneration, requesting the user to
re-run make and exit 1. And therefore do_compile goes kaboom.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>