foreign/*: add foreign folder

This folder now holds our subprojects and third-party code
foreign/pcie-driver: update submodule reference
kernel/*: restructure kernel folder

Now, the PCIe driver is located in foreign/pcie-driver
kernel/pcie-driver/Makefile: redirect targets to foreign/pcie-driver
Makefile: change KERNEL_DIR variable

Now, our PCIe driver resides on a different path
foreign/libmdp: move majordomo lib to foreign folder