Revert "seL4platsupport: Use strtol instead of sscanf to parse hex"

This reverts commit f61c0d582b3d2406317ff0276e8e7901105502b8.

The string being parsed is created using the %p format type to sprintf, as such it should
be parsed back with precisely the same format arguments.

libsel4platsupport/src/bootinfo.c

diff --git a/libsel4platsupport/src/bootinfo.c b/libsel4platsupport/src/bootinfo.c
index a011f33..2906035 100644
--- a/libsel4platsupport/src/bootinfo.c
+++ b/libsel4platsupport/src/bootinfo.c
@@ -23,8 +23,8 @@
         ZF_LOGE("Attempted %s in an environment without bootinfo.", __FUNCTION__);
         return NULL;
     }
-    void *bi = (void *)strtol(bi_string, NULL, 0);
-    if (bi == 0) {
+    void *bi;
+    if (sscanf(bi_string, "%p", &bi) != 1) {
         ZF_LOGE("bootinfo environment value '%s' was not valid.", bi_string);
         return NULL;
     }