coolpi-cm5: fix uboot patch dir

[amazingfate]

Description

Coolpi cm5 is using mainline u-boot with BOOTPATCHDIR unset because it doesn't want patches. And the uboot patch dir fallback to u-boot-rockchip64, which has a lot of patches for rockchip64 families.
Now we declare a fake patch dir for uboot to pass build.

I find the two boards affected by command:

git grep "unset BOOTPATCHDIR"

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Please also note any relevant details for your test configuration.

• ./compile.sh uboot BOARD=coolpi-genbook BRANCH=edge DEB_COMPRESS=xz
• ./compile.sh uboot BOARD=coolpi-cm5 BRANCH=edge DEB_COMPRESS=xz

Checklist:

Please delete options that are not relevant.

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules