ar71xx: switch TP-Link RE450 v1 to dynamic partitioning

ar71xx: switch TP-Link RE450 v1 to dynamic partitioning

As mentioned in commit 5f24933 recent changes on ar71xx (switch to 4.14,
memory compaction, ...) cause an increase in kernel size, making it too
big for RE450.

RE450 images were not build due to the following error message:
os-image partition too big (more than 1572864 bytes): Success

Tested on RE450, device boots and was used to send this patch.

Reported-by: Enrico Mioso <mrkiko.rs@gmail.com>
Suggested-by: David Bauer <mail@david-bauer.net>
Signed-off-by: Radek Dostál <rd@radekdostal.com>
[rewrote commit msg keeping it tight + fixed SoB lines]
Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>