u-boot: update PKG_BUILD_DIR for version 2026.10+

Starting with version 2026.10, the folder name inside the tarball
has changed: an additional `v` is prepended to the directory name.

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: openwrt/openwrt#25778
Signed-off-by: Robert Marko <robimarko@gmail.com>

Author: DragonBluep

include/u-boot.mk

@@ -12,6 +12,11 @@ PKG_SOURCE_URL = \
 endif
 
 PKG_BUILD_DIR = $(BUILD_DIR)/$(PKG_NAME)-$(BUILD_VARIANT)/$(PKG_NAME)-$(PKG_VERSION)
+ifndef PKG_SOURCE_PROTO
+ifeq ($(firstword $(sort $(PKG_VERSION) 2026.10)),2026.10)
+  PKG_BUILD_DIR = $(BUILD_DIR)/$(PKG_NAME)-$(BUILD_VARIANT)/$(PKG_NAME)-v$(PKG_VERSION)
+endif
+endif
 
 PKG_TARGETS := bin
 PKG_FLAGS:=nonshared