[cpu] Rename cpu_id::validate() to cpu_id::features()

Validate wasn't a correct name anymore. Also move the features enum out
of the cpu_id class scope and into the `cpu` namespace directly.

src/boot/hardware.cpp

@@ -79,7 +79,7 @@ check_cpu_supported()
     status_line status {L"Checking CPU features"};
 
     cpu::cpu_id cpu;
-    cpu::cpu_id::features features = cpu.validate();
+    cpu::features features = cpu.features();
     bool supported = true;
 
 #define CPU_FEATURE_OPT(...)