cpu/arm7_common: hook up puf_sram

puf_sram only relies on an uninitialized chunk of memory.
This means to enable it we just have to hook up puf_sram_init().

All memory after __bss_end should be uninitialized at startup, so
just use that.

cpu/arm7_common/Makefile.features

@@ -3,3 +3,4 @@ FEATURES_PROVIDED += arch_arm
 FEATURES_PROVIDED += arch_arm7
 FEATURES_PROVIDED += cpp
 FEATURES_PROVIDED += periph_pm
+FEATURES_PROVIDED += puf_sram

cpu/arm7_common/arm7_init.c

@@ -20,6 +20,9 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include "thread.h"
+#ifdef MODULE_PUF_SRAM
+#include "puf_sram.h"
+#endif
 
 #include "log.h"
 
@@ -63,6 +66,12 @@ void bootloader(void)
     /* initialize bss and data */
     _init_data();
 
+#ifdef MODULE_PUF_SRAM
+    /* uninitialized heap starts after bss section */
+    extern unsigned int __bss_end;
+    puf_sram_init((uint8_t *) __bss_end, SEED_RAM_LEN);
+#endif
+
     /* cpu specific setup of clocks, peripherals */
     cpu_init();
 

cpu/lpc2387/include/cpu_conf.h

@@ -71,6 +71,11 @@ extern "C" {
 #define CC_CONF_WARN_UNUSED_RESULT      __attribute__((warn_unused_result))
 /** @} */
 
+/**
+ * @brief   Attribute for memory sections required by SRAM PUF
+ */
+#define PUF_SRAM_ATTRIBUTES __attribute__((used, section(".noinit")))
+
 #ifdef __cplusplus
 }
 #endif