buildroot: specify the compilation directory

Signed-off-by: carbon <carbon@milkv.io>

buildroot-2024.02/Makefile

@@ -34,8 +34,12 @@ SHELL := $(shell if [ -x "$$BASH" ]; then echo $$BASH; \
 # or avoid confusing packages that can use the O=<dir> syntax for out-of-tree
 # build by preventing it from being forwarded to sub-make calls.
 ifneq ("$(origin O)", "command line")
+ifneq ($(TARGET_OUTPUT_DIR),)
+O := $(TARGET_OUTPUT_DIR)
+else
 O := $(CURDIR)/output
 endif
+endif
 
 # Check if the current Buildroot execution meets all the pre-requisites.
 # If they are not met, Buildroot will actually do its job in a sub-make meeting