MFC r308477:

make pxeboot consistent with common/dev_net.c

Always define boot.netif.server in kenv in pxeboot
Add "boot.tftproot.server" to kenv when pxeboot uses tftpfs
Change the code order when setting env for TFTP or NFS to be the same as
common/dev_net.c

Reported by: tsoome