support/cmake: impersonate a Linux system even more

support/cmake: impersonate a Linux system even more

Some packages test the system name to decide whether to enable/disable
features or link with specific libs.

So we forcefully set the system name form our custom system file, so
that packagses still believe they are running on Linux rather than
Buildroot.

Fixes:
    fastd      : http://autobuild.buildroot.net/results/f1d/f1dfe90068ad62e733f17a22202235415bda3974/
    paho-mqtt-c: http://autobuild.buildroot.net/results/457/457d76279e16247bf58c838a2c5dd0a4f3962c21/
    libiio     : http://autobuild.buildroot.net/results/281/2812b008a0ab6bab5fe4d45eb9ffe4e9496a8cb4/
    and so on...

Reported-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reported-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Ben Boeckel <mathstuf@gmail.com>
Cc: Jörg Krause <joerg.krause@embedded.rocks>
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>