Set BOARD_KERNEL_CMDLINE to +=

This allows us to set additional properities for testing/development in
vendor projects.

Test: manual
Bug: 122362317
Change-Id: If547cf39446327b6a2d13d85e1468f65000fd57c
diff --git a/marlin/BoardConfig.mk b/marlin/BoardConfig.mk
index e1da54d..48af1d9 100644
--- a/marlin/BoardConfig.mk
+++ b/marlin/BoardConfig.mk
@@ -92,7 +92,7 @@
 TARGET_USES_QCOM_BSP := true
 endif
 
-BOARD_KERNEL_CMDLINE := console=ttyHSL0,115200,n8 androidboot.console=ttyHSL0 androidboot.hardware=marlin user_debug=31 ehci-hcd.park=3 lpm_levels.sleep_disabled=1 cma=32M@0-0xffffffff loop.max_part=7
+BOARD_KERNEL_CMDLINE += console=ttyHSL0,115200,n8 androidboot.console=ttyHSL0 androidboot.hardware=marlin user_debug=31 ehci-hcd.park=3 lpm_levels.sleep_disabled=1 cma=32M@0-0xffffffff loop.max_part=7
 
 BOARD_ROOT_EXTRA_FOLDERS := firmware firmware/radio persist
 BOARD_ROOT_EXTRA_SYMLINKS := /vendor/lib/dsp:/dsp