linux: update to  linux-3.15.7

Signed-off-by: Stephan Raue <stephan@openelec.tv>
- libnfs: update to 1.9.5

by upgrading we get new features like directory caching and file read-
ahead support.
projects/RPi/patches/linux: update RPi support patch

Signed-off-by: Stephan Raue <stephan@openelec.tv>