tfa-layerscape: fix build on systems without openssl headers

tfa-layerscape: fix build on systems without openssl headers

The build fails when the openssl/sha.h header file is not installed on
the host system. Fix this by setting the HOSTCCFLAGS variable to the
OpenWrt HOST_CFLAGS variable, without setting this the include paths and
other modifications in the host flags done by OpenWrt will be ignored by
the build.

This fixes the following build problem:
gcc -c -D_GNU_SOURCE -D_XOPEN_SOURCE=700 -Wall -Werror -pedantic -std=c99 -O2 -I../../include/tools_share fiptool.c -o fiptool.o
In file included from fiptool.h:16,
                 from fiptool.c:19:
fiptool_platform.h:19:11: fatal error: openssl/sha.h: No such file or directory
   19 | # include <openssl/sha.h>
      |           ^~~~~~~~~~~~~~~

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>