beryllium.mk: Drop unspported 5.4 kernel reference

Fixes: b7c457bd2f20 ("prebuilt-kernel: Remove android-5.4 prebuilts")
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
diff --git a/beryllium.mk b/beryllium.mk
index 8fd654d..a8d5b9b 100644
--- a/beryllium.mk
+++ b/beryllium.mk
@@ -1,5 +1,5 @@
 ifndef TARGET_KERNEL_USE
-TARGET_KERNEL_USE := 5.4
+TARGET_KERNEL_USE := mainline
 endif
 
 KERNEL_MODS := $(wildcard device/xiaomi/beryllium/prebuilt-kernel/android-$(TARGET_KERNEL_USE)/*.ko)