Specify kernel.page_shift in bootconfig

Updated microdroid kernel prebuilt contains the
"ad60adea543b560a456b2d7ac31408d9c82fe102 ANDROID: 16K: Init page_shift
param in a pure_initcall()" patch, which adds support for specifying
kernel.page_shift in the bootconfig.

Bug: 376901009
Test: presubmit
Test: atest MicrodroidTests on aosp_cf_x86_64_phone_pgagnostic
Change-Id: Iaefaee7d4fa755884bb7ab767664f984d17c1166
diff --git a/build/microdroid/bootconfig.x86_64_16k b/build/microdroid/bootconfig.x86_64_16k
index ee01de5..964a5fc 100644
--- a/build/microdroid/bootconfig.x86_64_16k
+++ b/build/microdroid/bootconfig.x86_64_16k
@@ -1 +1 @@
-page_shift = 14
+kernel.page_shift = 14