efibootmgr: backport upstream patch to fix build with gcc 6.x

efibootmgr: backport upstream patch to fix build with gcc 6.x

efibootmgr was built with -fshort-wchar, which causes gcc to use
2-byte wchar_t instead of the normal 4-byte wchar_t. But this doesn't
work anymore with gcc 6.x, as it causes a build failure:

/home/test/autobuild/run/instance-3/output/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-gnueabihf/6.3.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: warning: efibootmgr.o uses 2-byte wchar_t yet the output is to use 4-byte wchar_t; use of wchar_t values across objects may fail
/home/test/autobuild/run/instance-3/output/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-gnueabihf/6.3.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: warning: efi.o uses 2-byte wchar_t yet the output is to use 4-byte wchar_t; use of wchar_t values across objects may fail
/home/test/autobuild/run/instance-3/output/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-gnueabihf/6.3.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: warning: unparse_path.o uses 2-byte wchar_t yet the output is to use 4-byte wchar_t; use of wchar_t values across objects may fail

Upstream solution was do simply drop -fshort-wchar:

  https://github.com/rhinstaller/efibootmgr/commit/3466fd05c8c6f1052e0426d64eed40f8a88fd78f

So this commit simply backports this upstream patch.

Fixes:

  http://autobuild.buildroot.net/results/87c3ff2427331eb9ab31c9eb685fe80954a0a98f/

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