[prefix] Use .bss16 as temporary stack space for calls to install_block

Some decompression algorithms (e.g. LZMA) require large amounts of
temporary stack space, which may not be made available by all
prefixes.  Use .bss16 as a temporary stack for the duration of the
calls to install_block (switching back to the external stack before we
start making calls into code which might access variables in .bss16),
and allow the decompressor to define a global symbol to force a
minimum value on the size of .bss16.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/scripts/i386.lds

@@ -26,6 +26,13 @@ SECTIONS {
 
     PROVIDE ( _max_align = 16 );
 
+    /*
+     * Allow decompressor to require a minimum amount of temporary stack
+     * space.
+     *
+     */
+    PROVIDE ( _min_decompress_stack = 0 );
+
     /*
      * The prefix
      *
@@ -87,6 +94,7 @@ SECTIONS {
 	*(.bss16.*)
 	*(.stack16)
 	*(.stack16.*)
+	. = MAX ( ., _mdata16 + _min_decompress_stack );
 	_edata16 = .;
     }
     _data16_filesz	= ABSOLUTE ( _mdata16 ) - ABSOLUTE ( _data16 );