Set BOARD_KERNEL_CMDLINE to +=

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

Bug: 122362317
Test: manual, build sailfish and svelte target, no behavior change
Change-Id: I6b2a608a79726fb53dc1f224ef4fd6713684aa02
diff --git a/sailfish/BoardConfig.mk b/sailfish/BoardConfig.mk
index 6cc647b..dca1b68 100644
--- a/sailfish/BoardConfig.mk
+++ b/sailfish/BoardConfig.mk
@@ -90,7 +90,7 @@
 TARGET_USES_QCOM_BSP := true
 endif
 
-BOARD_KERNEL_CMDLINE := console=ttyHSL0,115200,n8 androidboot.console=ttyHSL0 androidboot.hardware=sailfish 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=sailfish 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