Makefile: Only look for @MARKER@ at the start of a line when generating libgrub_a_ini...

Makefile: Only look for @MARKER@ at the start of a line when generating libgrub_a_init.lst

Under certain conditions libgrub.pp gets generated with a such that it
contains a bunch of CPP defines, at least one of which contains "@MARKER@".
This line should not be used when generating libgrub_a_init.lst, otherwise
we get compiler errors like:

  libgrub_a_init.c:22:18: error: stray ‘#’ in program
     22 | extern void grub_#define_init (void);
        |                  ^
  libgrub_a_init.c:22:19: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘define_init’
     22 | extern void grub_#define_init (void);
        |                   ^~~~~~~~~~~
  libgrub_a_init.c:23:18: error: stray ‘#’ in program
     23 | extern void grub_#define_fini (void);
        |                  ^
  libgrub_a_init.c:23:19: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘define_fini’
     23 | extern void grub_#define_fini (void);
        |                   ^~~~~~~~~~~
  ...

When generating libgrub_a_init.lst only lines starting with "@MARKER@"
are desired.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>