build/debian: Disable ballooning on images with a generic kernel

As described in the linked bug report, enabling memory ballooning
on a generic kernel (without our patches) results in strange
corruption issues on Arm64.

In this CL, we update our build script to:
1. make building with our custom kernel the default build option,
2. disable ballooning on Arm64 images with unpatched generic kernel.

Bug: 396233393
Test: Deployed and tested on Komodo:
Test: - ./build_in_container.sh -a aarch64
Test: - ./build_in_container.sh -g -a aarch64

Change-Id: I4441dbf5886675251836ed772eaa804d0520bea9
diff --git a/build/debian/build_in_container.sh b/build/debian/build_in_container.sh
index e3adcae..739d2dd 100755
--- a/build/debian/build_in_container.sh
+++ b/build/debian/build_in_container.sh
@@ -6,7 +6,7 @@
   echo "Options:"
   echo "-h         Print usage and this help message and exit."
   echo "-a ARCH    Architecture of the image [default is host arch: $(uname -m)]"
-  echo "-k         Build and use our custom kernel [default is cloud kernel]"
+  echo "-g         Use Debian generic kernel [default is our custom kernel]"
   echo "-r         Release mode build"
   echo "-s         Leave a shell open [default: only if the build fails]"
   echo "-w         Save temp work directory in the container [for debugging]"
@@ -18,17 +18,17 @@
 save_workdir_flag=
 shell_condition="||"
 
-while getopts "a:hkrsw" option; do
+while getopts "a:ghrsw" option; do
   case ${option} in
     a)
       arch="$OPTARG"
       ;;
+    g)
+      kernel_flag="-g"
+      ;;
     h)
       show_help ; exit
       ;;
-    k)
-      kernel_flag="-k"
-      ;;
     r)
       release_flag="-r"
       ;;