py/py.mk: Don't hardcode path to libaxtls.a.

Use -L$(BUILD), not -Lbuild. Otherwise, builds for different archs/subarchs
using different values of BUILD may fail.

py/py.mk

@@ -26,7 +26,7 @@ ifeq ($(MICROPY_PY_USSL),1)
 CFLAGS_MOD += -DMICROPY_PY_USSL=1
 ifeq ($(MICROPY_SSL_AXTLS),1)
 CFLAGS_MOD += -DMICROPY_SSL_AXTLS=1 -I$(TOP)/lib/axtls/ssl -I$(TOP)/lib/axtls/crypto -I$(TOP)/lib/axtls/config
-LDFLAGS_MOD += -Lbuild -laxtls
+LDFLAGS_MOD += -L$(BUILD) -laxtls
 else ifeq ($(MICROPY_SSL_MBEDTLS),1)
 # Can be overridden by ports which have "builtin" mbedTLS
 MICROPY_SSL_MBEDTLS_INCLUDE ?= $(TOP)/lib/mbedtls/include