Makefile/linux: follow symlinks during fw enumeration for B2C_FW

Nvidia's firmware has complete folders being symlinked between different codenames:

nvidia/ga106/gsp -> ../ga102/gsp

This breaks # B2C_FW+=nvidia/ga106/* as it would try to add gsp as if it were a file rather than following the symlink and finding the following files:

-rw-r--r-- 1 root root    33114 Feb  1 10:25 booter_load-535.113.01.bin.zst
-rw-r--r-- 1 root root    18906 Feb  1 10:25 booter_unload-535.113.01.bin.zst
-rw-r--r-- 1 root root    16463 Feb  1 10:25 bootloader-535.113.01.bin.zst
-rw-r--r-- 1 root root 26287047 Feb  1 10:25 gsp-535.113.01.bin.zst

Fix this by telling find to follow symlinks during enumeration. This will however end up growing the size of the image drastically as the same file may be included multiple times (although compression should reduce duplication a bit).