kexec: fix build when BR2_STRIP_none=y

kexec: fix build when BR2_STRIP_none=y

When BR2_STRIP is set to none, a dummy 'true' app is used instead of
strip utility. However, kexec package always requires a real strip
executable as it is used to generate intermediate files during the build
process.

If 'true' is used the build system runs the following command:

true --strip-debug -o purgatory/purgatory.ro purgatory/purgatory.ro.sym

which obviously doesn't do anything useful and the build later fails
with:

bin/bin-to-hex purgatory < purgatory/purgatory.ro > kexec/purgatory.c
/bin/sh: purgatory/purgatory.ro: No such file or directory
kexec/Makefile:7: recipe for target 'kexec/purgatory.c' failed
make[2]: *** [kexec/purgatory.c] Error 1

To resolve it always specify STRIP=$(CROSS_COMPILE)strip, regardless of
buildroot configuration.

[Peter: add a comment explaining why this is done]
Signed-off-by: Marcin Nowakowski <marcin.nowakowski@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>