trusted-firmware-a: fix build error when using ccache

When ccache is enabled trusted-firmware-a recipe fails with this
error message:

    make: *** No rule to make target 'aarch64-poky-linux-gcc'. Stop.

ccache prefix CC variable with 'ccache' word before compiler. Because
there are no quotes assigned to CC, only 'ccache' is assigned. The
compiler becomes a make target, producing the build error.

Add single quotes to LD is a good measure to prevent this kind of error.

Signed-off-by: Javier Tia <javier.tia@linaro.org>
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm/recipes-bsp/trusted-firmware-a/trusted-firmware-a.inc

@@ -101,9 +101,9 @@ def remove_options_tail (in_string):
     from itertools import takewhile
     return ' '.join(takewhile(lambda x: not x.startswith('-'), in_string.split(' ')))
 
-EXTRA_OEMAKE += "LD=${@remove_options_tail(d.getVar('LD'))}"
+EXTRA_OEMAKE += "LD='${@remove_options_tail(d.getVar('LD'))}'"
 
-EXTRA_OEMAKE += "CC=${@remove_options_tail(d.getVar('CC'))}"
+EXTRA_OEMAKE += "CC='${@remove_options_tail(d.getVar('CC'))}'"
 
 # Verbose builds, no -Werror
 EXTRA_OEMAKE += "V=1 E=0"