linux-yocto-custom: Clarify defconfig usage

It is necessary to supply file://defconfig to the SRC_URI when using
a defconfig (it is not implicitly understood as the commentary might
currently suggest).

(From OE-Core rev: 3e244e0e9c10438c2236e56b3de664d4560791f0)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
CC: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta-skeleton/recipes-kernel/linux/linux-yocto-custom.bb

@@ -13,7 +13,8 @@
 #
 #   You must also provide a Linux kernel configuration. The most direct
 #   method is to copy your .config to files/defconfig in your layer,
-#   in the same directory as the bbappend.
+#   in the same directory as the bbappend and add file://defconfig to
+#   your SRC_URI.
 #
 #   To use the yocto kernel tooling to generate a BSP configuration
 #   using modular configuration fragments, see the yocto-bsp and