linux-libc-headers: provide 3.0.x and 3.1 headers

linux-libc-headers: provide 3.0.x and 3.1 headers

Provide both a 3.0.x and a 3.1 set of headers to the toolchain.
Compatibility is maintained with older 2.6 headers by creating a
new variable that changes the SRC_URI based on the major version
number of the kernel.

Built and booted with 2.6.37.2, 3.0.8 and 3.1 linux-libc-headers.

(From OE-Core rev: 13c233ce4551542481d7a6390ff2119671137b95)

Signed-off-by: Bruce Ashfield <bruce.ashfield@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>