mpc85xx: p1010: Use zImage loader for Watchguard Firebox T10

mpc85xx: p1010: Use zImage loader for Watchguard Firebox T10

Since kernel 6.1, the Watchguard Firebox T10 can't boot anymore due to
increased kernel size.

This commit introduces the zImage loader from 7d768a9 to boot the kernel.
This is required, since the U-Boot version used in this device appears
to have a hard limit of 16MB for the kernel size it can handle. The
current kernel size is around 17MB, though, due to kernel page alignment
required for memory protection.

(cherry-picked from commit be7aa5bda40ee05b1dd0b4f4a1bf88e653ef875a)
Signed-off-by: Shine <4c.fce2@proton.me>
Link: https://github.com/openwrt/openwrt/pull/21130
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>