.gitignore: Split into subdirectory files

There's no need for the global list of files to ignore, so use git's
ability to work with more local configuration.

Change-Id: I50882e6756cbc0fdfd899353cc23962544690fb3
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/46879
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Christian Walter <christian.walter@9elements.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/external/.gitignore

@@ -0,0 +1,10 @@
+depthcharge/depthcharge/
+FILO/filo/
+GRUB2/grub2/
+LinuxBoot/linuxboot/
+SeaBIOS/seabios/
+tianocore/tianocore/
+tint/tint/
+U-Boot/u-boot/
+Memtest86Plus/memtest86plus/
+iPXE/ipxe/