tools: Add the network install public key

For reference, add the Raspberry Pi Network Install public key
which is compiled into the bootmain executable.

To verify the current image:

git clone https://github.com/raspberrypi/rpi-eeprom
wget https://downloads.raspberrypi.com/net_install/boot.sig
wget https://downloads.raspberrypi.com/net_install/boot.img
cd rpi-eeprom
./rpi-eeprom-digest -k imager/net_install_pubkey.pem -i ../boot.img -v ../boot.sig

On success the expected output is "Verified OK" and exit status 0.

See: #711

imager/net_install_pubkey.pem

@@ -0,0 +1,9 @@
+-----BEGIN PUBLIC KEY-----
+MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAno9f6RGBaf2yaWTwf8+y
+MO4snzEgLOr8L3S28PZBdyx2qqNrzZ+xUOfLNYI5CwvBgOyKtm5L+wJIR8NUqgBl
+tvtBMAy2Auh0Qe679vN6UnsWE/o3pCgFPdmH+EcKfPqNBYODgfL+eumGM9Lo2dnl
+6P3JBR4Uy2E171k4D9Pj5rhO2K4jySYwbTuFR/drB2nDBrrsUW+SArnkTLvEGLY1
+nONW+AIBaqBxb+wjD/TMvVdsCWNXabcRqYM9DDgVzGePKuQdX5aBdgDnlbtDodyq
+FnLcRjqGE7nSQBQILwmGl5EvHMGa8d3/aLE6eXmMCakXyF1HPRv2lOecxNMO3xTF
+zwIDAQAB
+-----END PUBLIC KEY-----