board: phytec: phycore_imx8mp.env fix netboot issues

The "run netargs" command should come later in the "netboot" command
order when using dhcp since it sets the server and client ip addresses.
The previous order led to misconfigured kernel boot params and thus
kernel panic when serverip was not manually set.
Further, following Linux FHS 3.0, change the nfsroot default directory
to /srv/nfs.

Fixes: 60f64bec ("board: phytec: phycore_imx8mp: Add fec support")
Signed-off-by: Yannic Moog <y.moog@phytec.de>