pkg-config: add --with-sysroot option for compiled in default sysroot setting

Similar to the --with-pc-path option. It works just like the existing
PKG_CONFIG_SYSROOT_DIR environment variable, but compiled in.
The environment variable overrides this default setting if set.

This way we don't need to pass PKG_CONFIG_SYSROOT_DIR in the environment
when building for the target, and it is easier to reuse pkg-config outside
BR (E.G. for the SDK) without having to setup special environment
variables.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>