We are not properly passing down TARGET_CFLAGS and TARGET_CPPFLAGS and
this can be noticed with external toolchains with the following build
error:

mipsel-linux-gnu-gcc -Wall -Wextra -pedantic -Waggregate-return
-Wcast-align -Wcast-qual -Wdisabled-optimization -Wpointer-arith
-Wshadow -Wsign-compare -Wundef -Wwrite-strings -Wbad-function-cast
-Wdeclaration-after-statement -Wmissing-prototypes -Wnested-externs
-Wstrict-prototypes  -O2 -std=c99 -I../lua/ -fPIC   -c -o lpvm.o lpvm.c
lpvm.c:10:17: fatal error: lua.h: No such file or directory
 #include "lua.h"
                 ^
compilation terminated.
make[4]: *** [lpvm.o] Error 1

Re-use COPT to pass our TARGET_CFLAGS and TARGET_CPPFLAGS

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>