pkg-cmake.mk: refactor the toolchainfile.cmake generation

This patch introduces a toolchainfile.cmake.in template which is filled
by Buildroot.

Using a toolchainfile.cmake.in template file allows to avoid overloading
quoting and/or escaping and it becomes much more similar to the
resulting file.

This patch also cleans up the quoting style.

[Peter: drop stdin redirect as suggested by Thomas]
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>