Fix build error when $(O) starts with ./
Fix build error when $(O) starts with ./

When a target starts with './', $@ does not. Therefore we must not use
$@ to access a variable if its name is created from $(out-dir) which
might start with './'. We use a macro to make sure that
$(conf-mk-file-export) is evaluated immediately rather than when the
rule is executed. It is needed because ta/ta.mk may be included twice
with different values for $(conf-mk-file-export) (32-bit and 64-bit
dev kits).

Fixes the following issue:

$ make O=./build
<snip>
  GEN     build/export-ta_arm32/mk/conf.mk
/bin/bash: -c: line 0: syntax error near unexpected token `)'
/bin/bash: -c: line 0: `() >> build/export-ta_arm32/mk/conf.mk'
ta/ta.mk:120: recipe for target 'build/export-ta_arm32/mk/conf.mk' failed
make: *** [build/export-ta_arm32/mk/conf.mk] Error 1

Signed-off-by: Jerome Forissier <ced78d268b4c9a0a20a59a9feeebd2ad90da03d3@linaro.org>
Reviewed-by: Etienne Carriere <f21bc5b8f3ea1962eb45af359f1abea6578cab94@linaro.org>

pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os