freescale/imx6-sabresd: use global patch dir for U-Boot patches

The best practice is to use the BR2_GLOBAL_PATCH_DIR for patches against
U-Boot, not BR2_TARGET_UBOOT_CUSTOM_PATCH_DIR.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Tested-by: Vincent Stehlé <vincent.stehle@freescale.com>
Cc: Gilles Talis <gilles.talis@gmail.com>
Cc: Gary Bisson <bisson.gary@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>