buflib: Change buflib_available() and add buflib_allocatable().

buflib_allocatable() is what buflib_available() was before (it was in fact
simply renamed). It returns the largest contiguous block of memory. This
can be allocated and will definitely succeed, although larger allocations
may also succeed if the buffer can be compacted and shrinked.

buflib_available() now counts all free bytes, contiguous or not. This
better matches the description and how the caller use it.

Change-Id: I511e4eb5f4cf1821d957b3f4ef8a685ce40fe289
Reviewed-on: http://gerrit.rockbox.org/481
Reviewed-by: Thomas Martitz <kugel@rockbox.org>
Tested-by: Thomas Martitz <kugel@rockbox.org>

firmware/core_alloc.c

@@ -67,6 +67,11 @@ size_t core_available(void)
     return buflib_available(&core_ctx);
 }
 
+size_t core_allocatable(void)
+{
+    return buflib_allocatable(&core_ctx);
+}
+
 int core_free(int handle)
 {
     return buflib_free(&core_ctx, handle);