weston-init: Launch weston with WESTON_DISABLE_ATOMIC on musl/x86

weston-init: Launch weston with WESTON_DISABLE_ATOMIC on musl/x86

Since we enabled drm/kms backend for qemux86, it does not work with musl
fdbdev worked ok, we see this error

[18:58:45.628] launching '/usr/libexec/weston-desktop-shell'
[18:58:45.737] atomic: couldn't commit new state: Invalid argument
[18:58:45.737] repaint-flush failed: Invalid argument

There seems to be some problem with atomics in libdrm, until that gets
diagnosed, simple solution is to not use it on musl when drm backend is used
thats why WESTON_DISABLE_ATOMIC=Y is set in environment file for such
cases

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>