os_detect: add support for Buildroot

Add support for systems generated with Buildroot:
https://buildroot.org/

Buildroot systems follows the freedesktop.org /etc/os-release spec. See:
https://git.busybox.net/buildroot/commit/?id=451a887894faddef019e9a0628c21b5f2e9eee56

Signed-off-by: Julien Olivain <juju@cotds.org>

src/rospkg/os_detect.py

@@ -711,6 +711,7 @@ def get_codename(self):
 
 OS_ALPINE = 'alpine'
 OS_ARCH = 'arch'
+OS_BUILDROOT = 'buildroot'
 OS_MANJARO = 'manjaro'
 OS_CENTOS = 'centos'
 OS_CYGWIN = 'cygwin'
@@ -743,6 +744,7 @@ def get_codename(self):
 
 OsDetect.register_default(OS_ALPINE, FdoDetect("alpine"))
 OsDetect.register_default(OS_ARCH, Arch())
+OsDetect.register_default(OS_BUILDROOT, FdoDetect("buildroot"))
 OsDetect.register_default(OS_MANJARO, Manjaro())
 OsDetect.register_default(OS_CENTOS, Centos())
 OsDetect.register_default(OS_CYGWIN, Cygwin())