Add sphinx as a build dependency and set C= to avoid needing the 'sparse' utility.
(From OE-Core rev: 20f3fcdf26bb167d7a91b9cbf037bbeb69beb189)
Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>