meson: always use meson subcommands

Invoking meson with just paths is a deprecated use of the binary and
causes warnings, so call 'meson setup' explicitly.

Whilst doing this cleanup, we can also call 'meson compile' and 'meson
install' instead of ninja.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>