extmod: Place lwIP memory in a dedicated section.

Place lwIP memory in its own section to allow to relocating it via
the linker script. If this section is not defined this falls through
to .bss. This macro also aligns the memory without adding trailing
padding bytes. See comments in src/include/lwip/arch.h

Signed-off-by: iabdalkader <i.abdalkader@gmail.com>

Author: iabdalkader

extmod/lwip-include/lwipopts_common.h

@@ -109,6 +109,11 @@
 // Needed for PPP.
 #define sys_jiffies sys_now
 
+// Place lwIP memory in a dedicated section to allow relocating it.
+// Note: alignment is enforced without adding trailing padding bytes.
+#define LWIP_DECLARE_MEMORY_ALIGNED(variable_name, size) \
+    __attribute__((section(".lwip"), aligned(MEM_ALIGNMENT))) u8_t variable_name[size]
+
 typedef uint32_t sys_prot_t;
 
 #endif // MICROPY_INCLUDED_LWIPOPTS_COMMON_H