deb-pkg: Fix building for MIPS big-endian or ARM OABI

These commands will mysteriously fail:

$ make ARCH=arm versatile_defconfig
[...]
$ make ARCH=arm deb-pkg
[...]
make[1]: *** [deb-pkg] Error 1
make: *** [deb-pkg] Error 2

The Debian architecture selection for these kernel architectures does
'grep FOO=y $KCONFIG_CONFIG && echo bar', and after 'set -e' this
aborts the script if grep does not find the given config symbol.

Fixes: 10f26fa6 ('build, deb-pkg: select userland architecture based on UTS_MACHINE')
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Michal Marek <mmarek@suse.cz>