Compile with '-Wmissing-include-dirs' flag

Add the '-Wmissing-include-dirs' flag to the CFLAGS and ASFLAGS
to make the build fail if the compiler or the assembler is given
a nonexistant directory in the list of directories to be searched
for header files.

Also remove 'include/bl1' and 'include/bl2' directories from the
search path for header files as they don't exist anymore.