uclibc: simplify UCLIBC_EXTRA_CFLAGS

Since commit f66952197b41452b77dfa065adc7172137e4c243 ("package/gcc:
remove gcc 4.8"), UCLIBC_EXTRA_CFLAGS no longer exists, and is
therefore always empty.

Passing the Buildroot make variable UCLIBC_EXTRA_CFLAGS into the
UCLIBC_EXTRA_CFLAGS fed into uClibc's make environment therefore no
longer makes sense. Therefore, just pass $(TARGET_ABI).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/uclibc/uclibc.mk

@@ -382,7 +382,7 @@ endif
 UCLIBC_MAKE_FLAGS = \
 	ARCH="$(UCLIBC_TARGET_ARCH)" \
 	CROSS_COMPILE="$(TARGET_CROSS)" \
-	UCLIBC_EXTRA_CFLAGS="$(UCLIBC_EXTRA_CFLAGS) $(TARGET_ABI)" \
+	UCLIBC_EXTRA_CFLAGS="$(TARGET_ABI)" \
 	HOSTCC="$(HOSTCC)"
 
 define UCLIBC_KCONFIG_FIXUP_CMDS