u-boot.mk: Fix building of recent U-Boot sources

u-boot.mk: Fix building of recent U-Boot sources

This patch fixes following error with U-Boot 2019.01 on imx6:

 In file included from tools/lib/crc16.c:1:0:
 ./tools/../lib/crc16.c: In function 'crc16_ccitt':
 ./tools/../lib/crc16.c:70:2: error: 'for' loop initial declarations are only allowed in C99 mode
   for (int i = 0;  i < len;  i++)
   ^
 ./tools/../lib/crc16.c:70:2: note: use option -std=c99 or -std=gnu99 to compile your code

Code was introduced in the upstream v2019.01-rc1-154-g51c2345:

 commit 51c2345bd24837f9f67f16268da6dc71573f1325
 Author: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
 Date:   Sun Nov 25 19:22:19 2018 +0100

    Roll CRC16-CCITT into the hash infrastructure

Upstream has added -std=gnu11 host flag in v2018.07-rc2-1-gfa89399:

 commit fa893990e9b53425af5f5059e04a2bffde91ccf9
 Author: Tom Rini <trini@konsulko.com>
 Date:   Tue Jun 19 23:53:54 2018 -0400

    Makefile: Ensure we build with -std=gnu11

Build tested on imx6: apalis, mx6sabresd, nitrogen6dl, nitrogen6dl2g,
                      nitrogen6q, nitrogen6q2g, nitrogen6s, nitrogen6s1g,
                      wandboard

Run tested: apalis (pending PR #1595)

Signed-off-by: Petr Štetiar <ynezz@true.cz>