loader: Add two new build knobs for x86 BIOS loaders

We have two sets of BIOS loaders: One that lives in stand/i386 and one
that lives in stand/userboot. Add knows to turn these on/off, with the
default being on. These often aren't needed when creating a minimal UEFI
system, so add knobs to turn them off.  Given light-weight VMs have
created a new use cases for these loaders, there's no plans at all to
eliminate them.

Sponsored by:		Netflix
Differential Revision:	https://reviews.freebsd.org/D58072

share/man/man5/src.conf.5

@@ -1,5 +1,5 @@
 .\" DO NOT EDIT-- this file is @generated by tools/build/options/makeman.
-.Dd July  7, 2026
+.Dd July  9, 2026
 .Dt SRC.CONF 5
 .Os
 .Sh NAME
@@ -1065,6 +1065,9 @@ Do not build LLVM target support for X86.
 The
 .Va LLVM_TARGET_ALL
 option should be used rather than this in most cases.
+.It Va WITHOUT_LOADER_BIOS
+Do not build x86 BIOS loader support
+
 .It Va WITHOUT_LOADER_BIOS_TEXTONLY
 Include graphics, font and video mode support in the i386 and amd64 BIOS
 boot loader.
@@ -1146,6 +1149,9 @@ arm/armv7 and powerpc/powerpc64.
 .It Va WITH_LOADER_USB
 Build the usb/kshim library
 
+.It Va WITHOUT_LOADER_USERBOOT
+Do not build x86 userboot loader support
+
 .It Va WITH_LOADER_VERBOSE
 Build with extra verbose debugging in the loader.
 May explode already nearly too large loader over the limit.

share/mk/src.opts.mk

@@ -125,6 +125,7 @@ __DEFAULT_YES_OPTIONS = \
     LLVM_BINUTILS \
     LLVM_BINUTILS_BOOTSTRAP \
     LLVM_COV \
+    LOADER_BIOS \
     LOADER_BIOS_TEXTONLY \
     LOADER_GELI \
     LOADER_IA32 \
@@ -133,6 +134,7 @@ __DEFAULT_YES_OPTIONS = \
     LOADER_OFW \
     LOADER_PXEBOOT \
     LOADER_UBOOT \
+    LOADER_USERBOOT \
     LOADER_ZFS \
     LOCALES \
     LOCATE \

stand/Makefile.amd64

@@ -1,5 +1,5 @@
-S.yes+=		userboot
-S.yes+=		i386
+S.${MK_LOADER_USERBOOT}+= userboot
+S.${MK_LOADER_BIOS}+=	i386
 
 SUBDIR_DEPEND_userboot+=	${INTERP_DEPENDS}
 # These won't get tacked on in an amd64 build

tools/build/options/WITHOUT_LOADER_BIOS

@@ -0,0 +1,2 @@
+Do not build x86 BIOS loader support
+

tools/build/options/WITHOUT_LOADER_USERBOOT

@@ -0,0 +1,2 @@
+Do not build x86 userboot loader support
+

tools/build/options/WITH_LOADER_BIOS

@@ -0,0 +1,2 @@
+Build x86 BIOS loader support
+

tools/build/options/WITH_LOADER_USERBOOT

@@ -0,0 +1,2 @@
+Build x86 userboot loader support
+