dde_ipxe_nic: add 'core/string.c' to 'SRC_C'

The 'string.c' file contains a 'memcmp()' implementation which is needed
by the driver.

Fixes #387.

dde_ipxe/lib/mk/dde_ipxe_nic.mk

@@ -5,7 +5,7 @@ LIBS = dde_kit dde_ipxe_support
 
 SRC_C = nic.c dde.c dummies.c
 
-SRC_C += $(addprefix core/, iobuf.c)
+SRC_C += $(addprefix core/, iobuf.c string.c)
 SRC_C += $(addprefix arch/x86/core/, x86_string.c)
 SRC_C += $(addprefix arch/i386/core/, rdtsc_timer.c)
 SRC_C += $(addprefix net/, ethernet.c netdevice.c nullnet.c eth_slow.c)

dde_ipxe/src/lib/dde_ipxe/dummies.c

@@ -20,7 +20,6 @@ void netdev_settings_operations() { TRACE; }
 void dbg_autocolourise(unsigned long id) { }
 void dbg_decolourise() { }
 void strerror() { TRACE; }
-int strcmp(const char *s1, const char *s2) { TRACE; return 0; }
 
 /* for eepro100.c */
 void init_spi_bit_basher() { TRACE; }