drivers/usart: fix wildcard CI build (kernel_config unresolved import)

userspace_build defaults True, so client/server were analyzed on the
host where kernel_config is no_config, causing E0432 in the CI
wildcard build. Gate on TARGET_COMPATIBLE_WITH instead.

drivers/usart/client/BUILD.bazel

diff --git a/drivers/usart/client/BUILD.bazel b/drivers/usart/client/BUILD.bazel
index 4b22336..744c074 100644
--- a/drivers/usart/client/BUILD.bazel
+++ b/drivers/usart/client/BUILD.bazel
@@ -1,16 +1,14 @@
 # Licensed under the Apache-2.0 license
 
 load("@rules_rust//rust:defs.bzl", "rust_library")
+load("//target/ast10x0:defs.bzl", "TARGET_COMPATIBLE_WITH")
 
 rust_library(
     name = "usart_client",
     srcs = ["src/lib.rs"],
     edition = "2024",
     tags = ["kernel"],
-    target_compatible_with = select({
-        "@pigweed//pw_kernel/userspace:userspace_build_enabled": [],
-        "//conditions:default": ["@platforms//:incompatible"],
-    }),
+    target_compatible_with = TARGET_COMPATIBLE_WITH,
     visibility = ["//visibility:public"],
     deps = [
         "//drivers/usart/api:usart_api",

drivers/usart/server/BUILD.bazel

diff --git a/drivers/usart/server/BUILD.bazel b/drivers/usart/server/BUILD.bazel
index 5473c6a..3d7bc32 100644
--- a/drivers/usart/server/BUILD.bazel
+++ b/drivers/usart/server/BUILD.bazel
@@ -1,6 +1,7 @@
 # Licensed under the Apache-2.0 license
 
 load("@rules_rust//rust:defs.bzl", "rust_library")
+load("//target/ast10x0:defs.bzl", "TARGET_COMPATIBLE_WITH")
 
 rust_library(
     name = "usart_server",
@@ -10,10 +11,7 @@
     ],
     edition = "2024",
     tags = ["kernel"],
-    target_compatible_with = select({
-        "@pigweed//pw_kernel/userspace:userspace_build_enabled": [],
-        "//conditions:default": ["@platforms//:incompatible"],
-    }),
+    target_compatible_with = TARGET_COMPATIBLE_WITH,
     visibility = ["//visibility:public"],
     deps = [
         "//drivers/usart/api:usart_api",