package/pkg-cmake: add CMAKE_SYSTEM_VERSION to toolchainfile.cmake

Quoting the CMake documentation:

  When the CMAKE_SYSTEM_NAME variable is set explicitly to enable cross
  compiling then the value of CMAKE_SYSTEM_VERSION must also be set
  explicitly to specify the target system version.

Thus, we should also set CMAKE_SYSTEM_VERSION in toolchainfile.cmake. It
is supposed to be set to the value of `uname -r` on the target. We don't
have that exact value available (unless we build the kernel), but the
value of BR2_TOOLCHAIN_HEADERS_AT_LEAST contains the (minimum) version
of the kernel it will run on, so it should be OK for all practical
purposes.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>